strike-slip faults featuring 7 to 8 km stratigraphic offsets. (Brisebois, 1981 ...... Limestones Group (Early Devonian) west of Murdochville, Gasp6. Peninsula.
BULLETIN OF CANADIAN PETROLEUM GEOLOGY VOL. 49, NO. 2 (JUNE, 2001), P. 346-365
The Lower Devonian Upper Gasp6 Limestones in eastern Gasp6: carbonate diagenesis and reservoir potential DENIS LAVOIE, GUOXIANGCHI
MARTIN G. FOWLER Geological Survey of Canada (Calgary) 3303 - 33rd Street NW Calgary, AB T2L 2A7
Geological Survey of Canada (Quebec) 880 Chemin Sainte-Foy, C.P. 7500 Sainte-Foy, QC T2L 2A7
ABSTRACT The Lower Devonian Upper Gasp6 Limestones consist of the Forillon, Shiphead and Indian Cove formations. The depositional basin and stratigraphic succession were controlled by synsedimentary extension faults. Three major paleogeographic domains are recognized: a northern proximal outer shelf, a central distal outer shelf, and a southern toeof-slope. Numerous oil seeps occur in fractured limestones of the central domain, and limited hydrocarbon production has been recorded from these units. Primary porosity occurs in the northern domain in the Indian Cove Formation brachiopod-rich facies. Local meteoric dissolution enhancement of porosity is the result of late influx of meteoric waters, as inferred from the nature of fracture-filling calcite cements. Porosity enhancement is restricted to the northern domain, but this area is barren of hydrocarbons. Multiple episodes of burial fracturing, dissolution, and calcite cementation are found in the central and southern domains. Petrographic evidence indicates three distinct diagenetic events occurred before the development of vertical stylolites related to the Acadian Orogeny (Middle Devonian). Carbon (C) and oxygen (O) stable isotopes and fluid inclusion microthermometry of the fracture-filling calcite cements indicate significantly higher thermal conditions for the southern domain than the western sector of the central domain. Low thermal maturation and hydrocarbon occurrences in the eastern part of the central domain (e.g. the Mississippi Anticline area) have been documented. These observations are consistent with our new isotopic and fluid inclusion data, which suggests lower thermal conditions compared with adjacent areas. Highly fluorescent hydrocarbon fluid inclusions are present in fracture-filling calcite cement. API values average around 40 °, and GC-MS analysis of a limited volume of decrepitates suggests a Devonian or older marine black shale source rock.
R1~SUMg
Les Calcaires sup6rieurs de Gasp6 du D6vonien Infdrieur comprennent les formations de Forillon, Shiphead et Indian Cove. Le bassin de d6p6t et la succession stratigraphique r6sultante ont 6t6 controlds par des failles de tension syns4dimentaires d4limitant trois domaines pal4og6ographiques majeurs: une plate-forme proximale au nord, une plate-forme distale au centre et un environnement de pied de pente au sud. Des suintements d'huile et une production limit4e d'hydrocarbures sont connus dans les calcaires fractur4s du domaine central. Une porosit4 primaire fut conserv4e dans des facihs g brachiopodes de la Formation d'Indian Cove du domaine septentrional. Un accroissement du volume de la porosit4 fut reconnu localement et des circulations tardives d'eaux m4tdoriques en seraient responsables, ce qui est sugg6r6 par la nature des ciments de calcite remplissant les fractures. Cet 6v6nement est restreint au domaine septentrional, aucune migration d'hydrocarbure n'y est reconnue. De multiples phases de fracturation, de dissolution et de cimentation de calcite du domaine de l'enfouissement sont pr4sents dans les successions des domaines central et mdridional. La p6trographie sugg~re trois dv6nements diag6n~tiques distincts avant le d6veloppement de stylolites verticaux associds g l'orog6nie acadienne (Ddvonien M6dian). Les donnEes isotopiques du C et de 1'O et les donn6es microthermomEtriques d'inclusions fluides sur les calcites de remplissage de fractures indiquent des conditions thermiques plus 61ev6es pour le domaine mdridional et pour le secteur occidental du domaine central. Le segment oriental de ce dernier (e.g. la rdgion de l'Anticlinal de Mississippi) fut ant6rieurement document6 comme 6rant de maturation peu 61evde et est caract~ris6 par des accumulations
346
THE LOWER DEVONIAN UPPER GASPE LIMESTONES: CARBONATE D1AGENESIS AND RESERVOIR POTENTIAL
347
d'hydrocarbures. Ces faits concordent avec nos nouvelles donn6es isotopiques et d'inclusions fluides, lesquelles suggbrent des conditions thermiques plus basses que celles des r6gions adjacentes. Des inclusions flnides d'hydrocarbures fortement fluorescentes sont prdsentes dans les ciments de calcite remplissant des fractures. Les valeurs moyennes d'API sont aux environs de 40 et les analyses GC-MS de volumes limit6s d'huile provenant des inclusions suggbrent, comme roche mbre, un shale marin d6vonien ou plus vieux, fiche en mati~re organique. Traduit par les auteurs.
Organic matter diagenesis and clay mineralogy of the Upper INTRODUCTION The occurrence of oil seeps in the Lower Devonian Upper Gasp6 Limestones were systematically studied by Bertrand Gasp6 Limestones has attracted exploration drilling for more (1987, 1996). These thermal indicators suggest that, for the than a century in the northeastern part of the Gasp6 Peninsula, most part, the unit is within or close to the oil window in the but only a small volume of oil has been produced from these eastern part of the Gasp6 Peninsula (Bertrand and Malo, 2001, strata. Hydrocarbon seeps are typically located along faults this issue). In contrast, carbonate diagenesis is largely (McGerrigle, 1950). Previous work has documented that, in the unknown. The only previously published studies on carbonate areas where oil seeps are known (e.g. Mississippi Anticline diagenesis are those of Lavoie and Bergeron (1993) and Lavoie area; Fig. 1), the Upper Gasp6 Limestones are charactefised by (1993, 1994). The latter study describes the reconstructed starelatively uniform, fine-grained, distal outer-shelf facies. ble isotope composition of the Early Devonian Gasp6 Basin Primary porosity is close to nil, and hydrocarbons are stored in sea. The Upper Gasp6 Limestones are predominantly finely fractured reservoirs Lavoie, 1996a; Lavoie and Nadeau, 1996). crystalline, and little effort has been made to understand their
/
/
66 ~
65 °
St. Lawrence River
Sample Locations Northern domain 1. Forillon Peninsula 2. Road 132
Central domain 3. MissisippiAnticfine 4. Gait No.1 well 5. Sunny Bank well
Southern domain 6. Oatcake Creek J 7. Riviere St-Jean Anticline I 8. Bazire Creek 9. Grande R v ere
F(~rtin/T~rn'iscouata groups E~] Chaleurs Group Matapedia and Honorat/Cabano groups ~Z] Quebec Supergroup
D. LAVOIE, G. CH1 and M. FOWLER
348
diagenetic evolution. The tectonodiagenetic evolution of abundant calcite-filled and partially open fractures has been the focus of industry workers (Lavoie, 1996a, b; Lavoie and Nadeau, 1996; Lavoie and Chi, 1997). This paper describes the evolution of primary and secondary (fracture and dissolution) porosities in the Upper Gasp6 Limestones. Interpretations are based on petrographic, fluidinclusion, and O and C stable isotope data gathered from various cement fillings. Hydrocarbon charge is also described, based on the character of oil fluid inclusions in some cement phases. Because pore-filling cements are scarce, fracture-filling cements are focussed on in this study. GEOLOGICAL
SETTING
The study area, in the lower Paleozoic Gasp6 Belt (sensu Bourque et al., 1995), is located in the northeastern part of the Qurbec segment of the Appalachian Orogen (Bourque et al., 2001, Figs. 1, 2, this issue). The Gasp6 Mobile Belt comprises sedimentary rocks formed in numerous tectonic settings (Bourque et al., 1995). Carbonate sedimentation occurred during tectonic activity and quiescence, forming carbonate shelves that range in age from latest Ordovician to Early Devonian
--
381
i
Eifelian - -
~ •
(Bourque et al., 1986; Lavoie, 1992a, b). During the Early Devonian, this part of the Appalachian Orogen was located at about 18 ° south (Van der Voo, 1988). The Upper Gasp6 Limestones outcrop in the eastern part of the Silurian-Devonian Connecticut Valley-Gasp6 Synclinorium of the Gasp6 Belt (Bourque et al., 1995) (Fig. 1). The Connecticut Valley-Gasp6 Synclinorium lies between the Cambro-Ordovician Taconian allochthons to the north and the Middle Ordovician-earliest Silurian Aroostook-Perc6 Anticlinorium to the south (Bourque et al., 2001, this issue). The Connecticut Valley-Gasp6 Synclinorium is an Acadian (Middle Devonian) structure (Malo and Brland, 1989) dissected by a number of faults. In eastern Gasp6 Peninsula (Fig. 1), the Bassin Nord-Ouest and Troisi~me Lac faults are dextral strike-slip faults featuring 7 to 8 km stratigraphic offsets (Brisebois, 1981; Kirkwood, 1986). These structures are reactivated listric normal faults that were active in Early Devonian time (Roksandic and Granger, 1981; Bourque, 1990; Lavoie, 1992a; Malo and Bourque, 1993; Bourque, 2001, this issue; Malo, 2001, this issue). The Upper Gasp6 Limestones conformably overlie the Lower Silurian to Lower Devonian (Lochkovian) Chaleurs Group (Bourque et al., 2001, this issue). The Upper Gasp6 Limestones are conformably overlain by the Lower Devonian
Sea-levelcurve ( FALL
Upper Gasp@
RISE )
Limestones
t ~
...... ,'
Emsian -- 390--
I I I I I (~
Pragian In a
--
I
I-
~'l~
{Lesp~rance, 1980] {Achab et al., 1997
Peninsula ~
,, , ,, 8r:o / 8 i.,~, I I IIII lllll I
Gasp~ ~ Sandstones ~
386--
BRACHIOPODS CHITINOZOANS
{Forillon
I Elymothyris Zone
Illll
?Emsian
./
?eragian
UpperGasp6 ~ ~ I":l"=i":;J': .......... _.Li ..m ..est ...o.nes ................................
Rennseloeria Zone
396--
lower eragian low. eragian L up, p e r . o c n K o v l a n
Lochkovian
i
Chaleurs 409
Emsian
I
Group /
--
i
:"
1I00m
upper
Lochkovian
Pridolian 10
_=1
Modified from Bourque etal. {1995]
~]conglomerate IS]sandstone
~mudstone E~]carbonate
-~icalcale°us ~ dolomitlc ~ nodular
~ chert .. sandy - shaly
b=benthonicclay
Fig. 2. Upper Silurian-lower Middle Devonian stratigraphy of the eastern segment of the Gaspe Belt. The Upper Gaspe Limestones were deposited during the highstand of a second-order transgressive-regressive cycle. The detailed stratigraphic column of the Upper Gaspe Limestones is based on Lavoie (1992b) and refers to the northern domain succession in Forillon Peninsula. CG Mb = Cap Gaspe Member, MSA Mb = Mont St AIban Member.
349
THE LOWER DEVONIAN UPPER GASPt~"LIMESTONES: CARBONATE DIAGENESIS AND RESERVOIR POTENTIAL
(Emsian)-Middle Devonian (Eifelian) Gasp6 Sandstones (Bourque et al., 2001, this issue). In central Gaspr, the Upper Gasp6 Limestones are laterally equivalent to the siliciclastics of the Fortin Group, likely deposited below storm wave base (Lavoie, 1992a). The Upper Gasp6 Limestones constitute one of the rare carbonate units in the otherwise siliciclastic-dominated, post-Taconian succession (Bourque et al., 1995). These carbonates accumulated during a relative sea-level highstand within a 20 Ma transgressive-regressive cycle (Lavoie, 1993) (Fig. 2) comprising three tectonically controlled, third-order cycles (Fig. 3). Higher-order cyclic sedimentation has previously been documented empirically (Lavoie, 1992b) and statistically (Achab et al., 1997; Mussard and Lavoie, 1997). The Forillon Peninsula of eastern Gasp6 Peninsula (Fig. 1) is the type area of the Upper Gasp6 Limestones. There, three units are recognized by Lesprrance (1980): the Fofillon, Shiphead and Indian Cove formations. These lithostratigraphic units are
GASPI~
SANDSTONES
recognized throughout eastern Gaspr. However, south of the Bassin Nord-Ouest Fault (Figs. 1, 4), the succession is thicker and is chiefly a monotonous succession of lime mudstone and wackestone. South of the Troisirme Lac Fault (Fig. 1), the Upper Gasp6 Limestones are thicker and comprise impure limestone with abundant rotational and translational slide structures that interfinger with the coarse- to fine-grained siliciclastics of the Fortin Group (Rouillard, 1986; Lavoie, 1990, 1992b; Lavoie et al., 1990, 1991). Based on sedimentological analyses, three paleogeographical domains and associated facies are recognized, and their distribution was likely controlled by the Bassin Nord-Ouest and Troisi~me Lac faults. From northeast to southwest, they are (1) the northeastern proximal outer shelf domain; (2) the central, gently sloping, distal outer shelf domain; and (3) the southwestern slope and toe-of-slope domain (Fig. 4). Each of these domains contains a distinct facies belt that is described according to the paleogeographic
STORM DEPOSITS PACKSTONE/GRAINSTONE BEDS coarse.grained % Storm-beds Fall 0 10 20 30%
Rise
T ~""""'='=l'
A X I S 2 O F C,J~. ~, S M O O T H E D C O O R D I N A T E S
?all
Ri,~T
imP) 1i
~°
o
--___~ CHALEURS GROUP
[Indian Point Fm]
(a)
Mussard
Fig. 3. Cyclic third-order sedimentation recorded in the Upper Gasp6 Limestones. This cyclicity is empirically recognized on the basis of (A) lithofacies analyses (Lavoie, 1992b) and (B) statistical treatment (correspondance analysis = C.A.) of lithofacies data (Achab et al. 1997; and Lavoie 1997).
350
D. LAVOIE, G. CHI and M. FOWLER
setting. For example, domain (1) contains a proximal outershelf facies. Active faulting may have influenced the distribution of Upper Silurian to Lower Devonian sedimentation in the eastern part of the Gasp6 Belt (Amyot, 1984; Lavoie, 1992a; Malo and Bourque, 1993; Achab et al., 1997; Mussard and Lavoie, 1997; Bourque, 2001, this issue). The proximal outer shelf domain contains the shallowest water facies of Pragian age on the Gasp6 Peninsula (Lavoie, 1992b). There is a significant southward increase in thickness of the unit, from an average of 500 m in the northern, proximal outer shelf domain to nearly 2 km in the southernmost toe-of-slope domain (Lavoie, 1992a). The proximal outer shelf facies consists predominantly of siliceous to cherty lime rnudstone with subordinate bioclastic wackestone and bioclastic and intraclastic packstone-grain-
•...
@rlllon ,2.o
Perce
[~
Proximal outer shelf
0
so km
stone (storm beds) and sandy limestone (Lavoie 1992b; Mussard and Lavoie 1997). The distal outer shelf facies consists predominantly of impure to locally siliceous lime mudstone, with minor, thin bioclastic wackestone and packstone and some volcanics and volcaniclastics (Lavoie et al., 1991; Lavoie, 1992a, 1995). The toe-of-slope facies belt consists of impure lime mudstone interbedded with debris-flow units and coarse-grained siliciclastic beds (Lavoie et al., 1990; Lavoie, 1992a). STRATIGRAPHY FORILLON FORMATION
In the area north of the Bassin Nord-Ouest Fault, the Forillon Formation (Fig. 2) comprises two members: the Mont SaintAlban and the Cap Gasp6 members (Lavoie et al., 1990). The Mont Saint-Alban Member contains siliciclastic mudstone beds 3 to 50 cm thick with uncommon interbedded shaly lime mudstone and wackestone. The mudstones are poorly fossiliferous, although a few trilobites and brachiopods of Lochkovian age occur (Lesp6rance, 1980; Bourque et al., 1995). The Cap Gasp6 Member consists of wavy bedded, lime mudstone and wackestone beds 5 to 50 cm thick. These carbonates are slightly or pervasively silicified. The wackestone contains well-preserved macrofauna, including abundant trilobites and brachiopods of the Rennselaeria (Lower Devonian) Zone (Lesp6rance and Sheehan, 1975; Lesp6rance, 1980). Scattered packstone beds less than 50 cm thick also occur. South of the Bassin NordOuest Fault, the formation consists of a thick succession of argillaceous lime mudstone beds 5 to 15 cm thick, with thin shale partings.
Distal outer shelf
SHIPHEAD FORMATION ••
A'
GR
S l o p e / t o e of s l o p e
TL
BNO
^
I
•
0 I
2kin
North of the Bassin Nord-Ouest fault, the Shiphead Formation (Fig. 2) is a heterogeneous unit of interbedded impure carbonate (85%), fine- to coarse-grained siliciclastics (10%), and bentonitic clay (5%). Silty lime mudstone and wackestone occur in beds up to 50 cm thick. The macrofauna is similar to that of the Cap Gasp6 Member. Sandy packstone and grainstone interfinger with the lime mudstone and wackestone in the upper few metres of the unit. South of the Bassin NordOuest Fault, the formation is similar to the Forillon Formation, although with significantly more argillaceous and silty carbonates.
20 k m
INDIAN COVE FORMATION
In the northern facies belt, the Indian Cove Formation (Fig. 2) consists of siliceous to cherty lime mudstone and wackestone in wavy beds 10 to 35 cm thick. About 20% of the Chaleurs Group formation contains interbeds of thin-bedded argillaceous lime mudstone that are commonly silicified. The wackestone conFig. 4. Paleoenvironmental interpretation of lithotectonic domains of the Upper Gaspe Limestones in eastern Gaspe. The three depositional tains abundant fauna, mostly trilobites and brachiopods, includbelts are limited by Acadian (Middle Devonian) strike-slip faults. The ing species of the Etymothyris (Emsian) Zone in the uppermost schematic cross-section (A-A') shows the relationships between part of the formation (Lesp6rance, 1980). A significant volume synsedimentary faulting and lithofacies. Modified from Achab et al. (1997). BNO = Bassin Nord-Ouest Fault, TL = Troisieme Lac Fault, GR (about 20% of the unit) of cement- and allochem-rich packstone = Grande Rivi~re Fault. and grainstone beds (15 to 45 cm thick) occurs in the upper half Upper G a s p @ Limestones
~
Fortln Group
THE LOWER DEVONIAN UPPER GASPE LIMESTONES: CARBONATE DIAGENESIS AND RESERVOIR POTENTIAL
351
of the formation. South of the Bassin Nord-Ouest Fault, the for- picked for GC and GC-MS analyses. The veins were suffimation is a well-bedded, locally siliceous lime mudstone and ciently wide to allow elimination (by hand picking) of host wackestone (5 to 20 cm thick) with thin shale partings. rocks in the sample. Aliquots of dried powdered rock (about 100 mesh) were extracted using azeotropic chloroRESEARCH METHODS form:methanol (87:13) for 24 hours. The extracts were fracIn order to study the diagenetic evolution of the Upper tionated using open column chromatography (3/4 activated aluGaspE Limestones, specific outcrop sections and drill cores mina and 1/4 activated silica gel with an adsorbent:sample mass were described and sampled (Fig. 1). The representative sec- ratio of 100:1). GC and GC-MS analyses were made at the GSC tions were chosen from the three domains: the northern (Calgary) following the procedures described in Fowler et al. (Forillon Peninsula and Road 132), central (Mississippi (1995). Anticline, Gait No.1 Well and Sunny Bank Well), and southem (Oatcake Creek, Rivirre St-Jean Anticline, Bazire Creek, and Grande Rivibre) domains (Fig. 1). These sections are considPETROGRAPHY ered representative of the diverse lithotypes and regional diageCALCITE-FILLED BRACHIOPOD-RICHBEDS nesis of the Upper GaspE Limestones. Details of the sections Beds of packstone and locally grainstone, very rich in braare described in Lavoie et al. (1990, 1991) and Lavoie (1992a, chiopods, occur only in the northern proximal outer-shelf b, c). A total of 74 samples was collected (26 in the Forillon, 6 in domain (Road 132 and Forillon Peninsula, Fig. 1). They are the Shiphead Formation and 42 in the Indian Cove formations). restricted to the uppermost part of the Indian Cove Formation, Few samples were collected from the Shiphead Formation, in the top of metre-scale, shallowing-upward cycles (Lavoie, because the unit is poorly exposed. From these samples, 61 1992b; Achab et al., 1997). These beds consist of densely standard thin sections were made (22 for the Forillon, 5 for the packed shells in a muddy to sparry matrix (Fig. 5A) and show Shiphead and 34 for the Indian Cove). In addition, 30 polished visible porosity as intra-brachiopod cavities, partly filled by thin sections for cathodoluminescence (CL) petrography were various cements (Lavoie, 1992b, 1996b). The intergranular and intraparticular cements consist of nonprepared (14 for the Forillon, 2 for the Shiphead and 14 for the Indian Cove). Oxygen and carbon stable isotope analyses were ferroan calcite with, in the case of large pores, some later ferdone on 57 samples (13 for the Forillon, 3 for the Shiphead and roan calcite. A euhedral silica cement rarely caps this cement 41 for the Indian Cove). Finally, fluid inclusion studies were succession. The calcite cements occur as 0.1 to 1 mm blocky done on 11 double-polished thin sections (5 for the Forillon, 1 crystals that coarsen toward the centre of the pores (Fig. 5B). for the Shiphead and 5 for the Indian Cove). Hydrocarbon fluid CL petrography reveals that the non-ferroan calcite cement inclusions from two samples of the Indian Cove Formation consists of an initial, relatively thin, virtually isopachous rind of were analyzed using gas chromatography (GC) and gas chro- non- to blotchy-luminescent calcite coating the internal wall of matography-mass spectrometry (GCMS). the brachiopod shells (Fig. 5C). This primary pore-filling, nonPore- and fracture-filling cements were studied under light luminescent calcite (PF-NL) cement contains microdolomite and cathodoluminescence (CL) microscopy. Generations of inclusions and is likely a diagenetic low-magnesium calcite cements were differentiated based on crosscutting relationships after a precursor high-magnesium calcite precipitate (Lohmann and relationships with other diagenetic features, such as stylo- and Meyers, 1977), The PF-NL cement is conformably overlites. Samples for C and O isotope analyses were micro-drilled lain by zoned dull-bright luminescent calcite (PF-zoned) (Fig. from parent rock slabs, and integrity of individual cement phas- 5C). The later ferroan calcite is darker dull luminescent (PFes was further checked under CL for the milled areas. The car- dull). The PF-zoned and PF-dull cements are rhombus-shaped. bonate powders were treated at the Delta-Lab [Geological Well-preserved crystal tips pointing toward the centre of open Survey of Canada (GSC) (QuEbec)] following standard proce- voids can be seen locally, which indicate preservation of pridures. Results are expressed in the usual delta notation and mary pore space. In places, corrosion of crystal tips is observed, given in per nail relative to the VPDB standard. Precision of the which suggests some enlargement of pore space by dissolution data is better than _+0.1%ofor both 6180 and 613C. (Fig. 5C). Microthermometric measurements on fluid inclusions were carried out with a USGS heating/freezing stage made by Fluid Inc. The homogenization temperature (Th) and final ice-melting FRACTURE-FILLINGCALCITE CEMENTS temperatures (Tm-ice)of aqueous fluid inclusions were measured Fracture-filling calcite samples were collected from the three with a precision of _+I°C and _+0.2°C, respectively. Oil inclu- domains. Three distinct fracture systems and calcite fillings are sions were routinely described for fluorescence using a Zeiss II recognized in the samples from the distal outer shelf (central photomicroscope. Some fluorescence spectra (400-700 nm) of domain) and the slope and toe-of-slope (southern domain). oil inclusions were measured at the GSC (Calgary). Details of Another distinct system has been documented in the samples the method are described in Stasiuk and Snowdon (1997). Two from the proximal outer shelf (northern domain), although its calcite veins containing abundant oil inclusions were hand- time relationships with the other fracture systems is unclear.
352
D. LAVOIE, G. CHI and M. FOWLER
Northern Domain
The best developed fracture system occurs in the Indian Cove Formation, near the Forillon Peninsula. Samples were collected in order to document the relationship of fracturing and dissolution-enhanced primary porosity in the brachiopod-rich beds. Fractures are millimetre- to centimetre-scale and locally display irregular margins. From the limited number of samples, fractures appear to post-date stylolites. Fracture-filling calcite consists of large, blocky, cloudy crystals with small inclusionpoor crystals as final precipitates. No open pore space occurs in the sample studied. Under CL, the calcite cement consists of strongly zoned bright and dull luminescent idiomorphic crystals (Fig. 6). This calcite cement is called fracture-filling zoned calcite (FF-zoned). Central and Southern Domains Samples of these domains contain a complex network of fracture-fill cements associated with dissolution. A first fracture system consists of millimetre- to centimetre-sized fractures that developed mainly after bedding-parallel stylolites, but locally at the same time as the stylolites. The fractures are filled with large xenomorphic to idiomorphic cloudy calcite cements. Under CL, the cement is orange in luminescence (FF-orange) and is locally faintly zoned (Fig. 7A). In many places, dissolution followed this cement phase (Fig. 7B) with the resulting voids filled by later calcite cement (see below). The margin of the dissolution voids is locally the site of bitumen concentration. The second fracture set consists of millimetre- to centimetresized fractures that clearly post-date stylolite formation. Small idiomorphic, limpid (inclusion-poor) calcite cements fill fractures. Under CL, this cement, which is also present in dissolution voids cutting the calcite cement of the previous fracture set, consists of dull to very dark luminescent calcite (FF-dull), with some non-luminescent growth bands (Figs. 7, 8A). Again, in places, dissolution post-dates this calcite, with the resulting voids filled with a later calcite cement. The third and last fracture set is represented by rare submillimetre-sized fractures that post-date horizontal, but pre-date vertical, stylolites. Small, idiomorphic, inclusion-poor calcite crystals fill the fractures. Under CL, this cement, which also fills some dissolution voids in previous fractures, consists of bright luminescent calcite spar (FF-bright) (Fig. 8B). No visible void space is associated with this fracture set. Therefore, dissolution porosity was generated in the fracture-fill calcite cements. This porosity was either filled by various calcite cements or remained open; however, open space is uncommon and consists of millimetre- to rarely centimetre-sized cavities. Fig. 5. A. Brachiopod-rich packstone with some intrabrachiopod The degree of cavity connection is unknown, but local hydroopen pore space. Indian Cove Formation, Road 132 section, northern carbon storage and production suggest that a relatively efficient domain. B. Photomicrograph in plane-polarized light of intrabrachiopod pore-filling calcite cements showing an initial rind of inclusion-rich lamel- permeable framework is present, at least locally. lar radiaxial calcite (LRC) followed inward by coarse-crystalline inclusion-rich to inclusion-poor calcite spar. C. Photomicrograph under CL of intrabrachiopod pore-filling calcite cement showing the initial rind of nonto blotchy luminescent calcite (BLC) overlain by zoned, dull-bright luminescent calcite (PF-zoned). Corrosion of crystal tips is locally observed (arrows). Scale bars = 0.1 mm.
STYLOLITES
Stylolites are common in the Upper Gasp6 Limestones. More than 95% of all stylolites are either roughly parallel or slightly inclined to bedding planes, with the remaining oriented
THE LOWER DEVONIAN UPPER GASPr~ LIMESTONES: CARBONATE DIAGENESIS AND RESERVOIR POTENTIAL
at right angles to bedding planes. Most of the stylolites consist of small amplitude nonsutured seams. High amplitude sutured stylolites are particularly rare. Bedding-parallel stylolites pre-date the fractures, although some are coeval with the first phase of the burial fracture system. Bedding-perpendicular stylolites clearly post-date fractures and have been shown to act as migration conduits for some hydrocarbons (Bertrand, 1987, 1996).
353
those from the southern domain (Grande Rivibre and St. Jean River anticlines) range from -8.4 to -13.9%0. A similar trend is observed for the Indian Cove Formation (see below). SttIPHEAD FORMATION The results for the Shiphead Formation are shown in Table 1 and Figure 10. Only three analyses were made, all from the Mississippi Anticline samples. Two analyses of the
STABLE ISOTOPES Oxygen and carbon isotope analyses of the various cement phases are listed in Table 1. The range of d180 values is broad (-2.6 to -13.9%o), whereas that of d13C is relatively narrow (-1.5 to +3.2%o). Results for each of the three formations are discussed separately (Figs. 9-11). FORILLON FORMATION The results for the Forillon Formation are shown in Table 1 and Figure 9. Four analyses of the fracture-filling, orange luminescent calcite (FF-orange) yielded d180 values ranging from -5.1 to-8.7%~ (average: -6.9%o) and d13C values from +1.7 to +3.0%o (average: +2.5%0). Nine analyses of the fracture-filling, dull luminescent calcite (FF-dull) yielded dlSO values ranging from-5.5 to -13.9%o (average: -9.1%o) and dl3C values from +0.8 to +2.9%o (average: +2.3%0). When results are grouped by domains, an obvious southerly decrease in d180 values is noted (Fig. 9). The d180 values of FF-dull calcite range from -5.5 to -8.0%0 in the central domain (Mississippi Anticline, Galt and Sunny Bank wells), whereas
Fig. 6. Photomicrograph under CL of a fracture cutting through a lime mudstone of the Indian Cove Formation at Forillon Peninsula. The fracture is filled by strongly zoned bright and orange, dull luminescent calcite cement. Scale bar = 0.1 ram.
Fig. 7. A. Photomicrograph under CL of large idiomorphic calcite cement coating the wall of a fracture (arrows). The cement is orange, dull luminescent (FF-orange), faintly zoned, and overlain by large xenomorphic dull luminescent calcite cement. B. Photomicrograph under CL of orange, dull luminescent calcite crystals that have been intensely corroded and dissolved (arrows), with the resulting pore space being filled by a dark, dull luminescent calcite phase. Scale bars = 0.1 mm.
354
fracture-filling, orange luminescent calcite (FF-orange) yielded 8180 values of-6.6%o and -8.2%o with respective 813C values of +2.7%o and +2.6%0. A fracture-filling, dull luminescent calcite (FF-dull) provided a 8180 value of-7.3%o with a 813C value of +2.7%o. Although limited, the results from the Shiphead Formation agree with the range of relatively high 8180 values that characterize the central domain.
D. LAVOIE, G. CHl and M. FOWLER
INDIAN COVE FORMATION Samples for C and O isotope analyses for the Indian Cove Formation come from all three domains. In the northern domain, analyses were made on pore-filling calcite (PF-NL, PF-zoned, PF-dull) and fracture-filling calcite (FF-zoned). In the central and southern domains, fracture-filling calcites were analyzed (FF-orange and FF-dull). The results are listed in Table 1 and shown in Figure 11. Northern Domain
Three analyses of the non- and blotchy luminescent calcite cement (PF-NL) coating the walls of brachiopod shells have yielded 8180 values of -2.6, -2.9 and -4.2%0, with respective 813C values of +2.8, +2.0 and +1.5%o. These values are similar to those given by Lavoie (1993) from non-luminescent brachiopod shells which had 8180 values ranging from-2.7 to --4.6%o and 813C values from +1.4 to +1.7%o. Two analyses of the succeeding, strongly zoned luminescent calcite (PF-zoned) yielded ~180 values o f - 9 . 2 and -9.9%0, with respective 813C values of +1.3 and +1.2%o. Finally, three analyses of the following dull luminescent calcite cement (PF-dull) have 8180 values of-10.6, -10.8 and -11.1%o with 813C values of +0.8, +0.8 and +0.7%o, respectively. Four analyses of the fracture-filling, strongly zoned, bright, and dull luminescent calcite cement (FF-zoned) yielded 8180 values ranging from -11.2 to -12.5%o (average: -11.9%0 with 813C values from -1.5 to +0.3%o (average: -0.5%0). The depletion in 13C relative to the marine calcite field (Lavoie, 1993) suggests a source other than marine for bicarbonates. The low 8180 values suggest either precipitation at relatively high temperature or from fluids depleted in ~80. Central a n d S o u t h e r n D o m a i n s
Fourteen analyses of the orange luminescent calcite cement (FF-orange) from the central and southern domains yielded 8180 values from -4.7 to -11.3%o (average: -7.5%0 and 813C
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Fig. 9. Crossplot of 8180 and 813C values for the Forillon Formation. Fracture-filling orange luminescent calcite (FF-orange) samples are all from the central domain (Mississippi Anticline area). Results for fracturefilling dull calcite (FF-dull), grouped according to the sampling areas, show an obvious southerly decrease in 8180. The shaded box is the Early Devonian marine calcite field from Lavoie (1993).
355
THE LOWER DEVONIAN UPPER GASPI~ LIMESTONES: CARBONATE DIAGENESIS AND RESERVOIR POTENTIAL
from +0.6 to +3.2%o (average: +1.7%o). Twelve analyses of the values (Table 1 and Fig. 11). It is inferred that the FF-zoned caldull luminescent calcite cement yielded 6isO values ranging cite was precipitated in a mixing zone between meteoric water f r o m - 5 . 1 to -12.5%o (average: -8.7%0) and 513C values from and basinal fluids. +1.2 to +3.2%o (average: +2.1%o). The broad range in 6180 valCHARACTERISTICSOF OIL INCLUSIONSAND GEOBAROMETRY ues is partly related to locality. As in the Fofillon Formation, a A number of oil inclusions occur in FF-dull calcite in a samsoutherly decrease in 8180 values is observed, although there is ple from the Mississippi Anticline area in the central domain a large overlap between the central and the southern domains (UGL-33, Table 2) and in a sample from the Oatcake Creek (Fig. 11). area in the southern domain (UGL-57, Table 2). These oil inclusions are colourless under transmitted light (Fig. 14). FLUID INCLUSIONS Under blue light excitation, they strongly fluoresce. The fluoFluid inclusions were studied in the fracture-filling, strongly rescence colour ranges from blue-tinted white to white and yelzoned luminescent calcite (FF-zoned) from the northern lowish white. Fluorescence spectra of oil inclusions from UGLdomain, and in the orange (FF-orange) and dull (FF-dull) lumi- 33 show Lmax values from 430 to 500 nm and Q (= nescent calcites from the central and southern domains. Workable aqueous inclusions occur in most samples. Oil inclu~I~4VPDB sions were found in two samples from the Indian Cove Formation (one from the Mississippi Anticline area and central domain, and the other from the Oatcake Creek area or southern domain). The microthermometric results of both aqueous and ,ll, O oil inclusions from the Forillon and Indian Cove cements and O the geochemistry of oil inclusions are discussed in this section. MICROTHERMOMETRYOF AQUEOUS INCLUSIONS Fluid inclusions that are either isolated, randomly distributed, or occurring in clusters, are considered as primary or pseudo-secondary. The results of microthermometric measurements of aqueous inclusions are listed in Table 2. The overall range of homogenization temperatures (Th) is from probably
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361
THE LOWER DEVONIAN UPPER GASPt~ LIMESTONES." CARBONATE DIAGENESIS AND RESERVOIR POTENTIAL
calcite cements are both characterized by low 8180 values (-9.2 to -11.1%o) and relatively low (compared to fracture-fill calcite cement elsewhere) 813C values (+0.7 to +1.3%o). It is proposed that these last two cements were precipitated from meteoric water. This interpretation is supported by the stable isotopic and fluid inclusion data of the fracture-filling zoned calcite cement (FF-zoned) crosscutting the lithofacies, the former being characterized by very low 8180 (-11.2 to -12.4%o) and 813C (-1.5 to +0.3%0) values, and by the presence of fluid inclusions with very low salinity values (0.3 wt % NaCl-equivalent). It is likely that this late meteoric event, related to the lrnid-Devonian Acadian regional exposure (Bourque et al., 2001, this issue), was responsible for the local enhancement of the primary porosity of the brachiopod beds, although its exact timing could not be ascertained (Fig. 19). No evidence for hydrocarbon migration was found in the Indian Cove Formation samples of the northern domain.
CENTRAL AND SOUTHERN DOMAINS The central and southern domains include distal outer shelf and toe-of-slope settings (Lavoie, 1992a). For these domains, the fracture-filling calcite cements have been shown to be rather uniform petrographically. A first fracture event was almost coeval with the inception of compaction stylolites. The fractures were filled by orange-dull calcite cement (FF-orange). The ensuing fracture event was accompanied by some dissolution of previous fracture-filling calcite cements, and post-dates bedding-parallel stylolites. Open voids associated with this fracture event (both fracture and dissolution voids) were ahnost totally occluded by a dull calcite cement (FF-dull). Finally, a poorly expressed third fracture set, also associated with dissolution of previous cements, occurred before the formation of bedding-perpendicular stylolites. The voids associated with this fracture event were also virtually occluded by a bright luminescent calcite cement (FF-bright). This general pattern is consistent for all studied localities and only differs in the intensity of the various fracture events. However, major differences in geochemical characteristics of cements are obvious from one locality to the other, especially between the central and southern domains. Central Domain Samples included in this area are those from the Mississippi Anticline and from the Galt and Sunny Bank cores (Fig. 1). The diagenetic evolution of the limestone in this area is illustrated in Figure 20. The first two fracture-filling cements (FF-orange and FF-dull) consist of calcite derived from saline, marinederived brines precipitated at relatively low temperature (Th of aqueous inclusions
