Optimizing Field Development of Deep Cretaceous ...

SPE Society of Petroleum Engineers

SPE 23620 Optimizing Field Development of Deep Cretaceous Fractured Reservoirs, Lake Maracaibo, Venezuela R. Franseen, V.C. Vahrenkamp and W.J.E. Van der Graaff, Koninklijke/Shell Exploratie en Produktie Laboratorium. and P.J. Munoz, Maraven, SA This paper was presented at the Second Latin American Petroleum Engineering Conference, II LAPEC, of the Society of Petroleum Engineers held in Caracas, Venezuefa, March 8-11, 1992 This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the II LAPEC or the SPE and are subject to correction by the author(s). The material, as presented. does not necessarily reflect any position of the It LAPEC or the SPE, its officers, or members. Papers presented at SPE meetings are subject to pubflcation review by Editorial Committees of the Society of Petroleum Engineers. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Write Publications Mana"Or, SPF p.O.Box 833836 Richardson TX 75083-3836 U.S.A. Telex 730989 SPEDAL.

ABSTRACT

INTRODUCTION

The Deep Cretaceous carbonate reservoirs of Lake Maracaibo, Venezuela, produce mainly from openfractures_ Any improvement in production rate requires optimal access of the wellbore to open fractures. Well tracks with maximum rates of open fracture interception have been calculated using computer models for a structure defined by the Icotea fault in Block IX. Optimum well tracks are towards azimuth 330 0 with deviation 60° in the West Flcmk and towards azimuth 030 0 with deviation 60° in the East Flank.

The Cretaceous carbonate reservoirs in Lake Maracaibo are found beneath the productive Tertiary clastic reservoirs and are referred to as the "Deep Cretaceous", since they lie at a depth in excess of 10,000 ft (approx. 3000 m). Production of hydrocarbons from these Deep Cretaceous carbonate reservoirs is controlled by open fractures that connect the matrix and stylolite porosity with the wellbore 1 _ Any improvement in production rate requires effective access to the open, hydraulically conductive fractures and knowledge of their occurrence with respect to stratigraphic architecture. This will help to: 1) optimize the interception rate of the hydraulically conductive fractures 2 ; 2) predict the pressure sensitivity of the reservoir; and 3) establish the regional validity of the reservoir geological model.

The open fracture networks consist of incompletely cemented or leached fractures which contain a "channel and island" structure of interconnected porosity. This structure, in combination with the high rock matrix strength and the subparallel strike of the open fractures relative to the E-W trend of the present-day maximum horizontal stress, indicate that it is unlikely that the conductivity of individual fracture will be reduced drastically upon pressure depletion.

This contribution intends to show how a detailed description of the fracture network, based on core observations and the relationship of open-fracture development relative to the stratigraphic architecture, helps to optimize field development. Rock mechanical tests have been performed to assess the effect of reservoir pressure decline on the hydrauliC conductivity of the open fractures.

The reservoir geological model established for Block IX is consistent with data obtained from the West Flank of Block I, some 40 km to the north and therefore has regiona; validity.

References and illustrations at end of paper

43

2

OPTIMIZING THE FIELD DEVELOPMENT OF DEEP CRETACEOUS FRACTURED .. RESERVOIRS, LAKE MARACAIBO, VENEZUELA

SETTING

driller's depth for SVS-229. Fracture density was determined by counting the total number of fractures per one-foot interval. For fracture modelling purposes and porosity estimation the fracture density was expressed as the number of fractures per unit volume. The effective fracture porosity (Le. fracture pore space not occupied by cements and gouge) and internal fracture geometry was evaluated by measuring the open-fracture width both on thin sections and directly on the core.

Fractures occurring in two vertical cores taken from Wells SVS-225 and SVS-229 penetrating the Deep Cretaceous carbonate reservoirs in Block IX of Lake Maracaibo were studied. Wells SVS-225 and SVS-229 are respectively located on the East Flank and West Flank of an anticlinal structure parallel to the left-Iaterallcotea fault (Fig. 1). The structural geological history of this part of the Maracaibo Block is characterized by two phases of deformation. From the early Eocene till the late Oligocene, NW-SE to NNW-SSE compression 3 corresponded with left-lateral movement on NE to NNE trending strike slip faults (e.g. Icotea fault, Fig. 1). Subsequently, the NNE-wards translation of the Maracaibo Block along major strike slip faults, such as the Santa Marta and Bocono faults, started in late Tertiary times 4 (early Miocene). The present-day principal compression direction, determined from earthquake focal mechanisms, is around E-W5.

Induced fractures, resulting from coring, stress relaxation and core-shed diagenesis (drying out) were differentiated from natural fractures on the basis of their morphology (petal shapes, initiation point and plume structures9 ), lack of cement fillings and relative age with respect to natural fractures.

FRACTURE NETWORK DESCRIPTION Geometry and relative timing of fracture networks

The Cretaceous sedimentary sequence of western Venezuela comprises an overall transgressive sequence from basal conglomerates (La Quinta Formation), coasta~ sandstones (Rio Negro Formation), shallow marine carbonates (Cogollo Group) and deep marine carbonates (La Luna Formation) overlain by a regressive, predominantly shaley sequence6 . The core of Well SVS-225 covered parts of the Rio Negro sandstone and the lower part of the Cogollo Group; the core of Well SVS-229 covered the top of the Cogollo Group (Maraca Formation) and the lower part of the La Luna Formation (Fig. 1).

The prediction of optimum well paths for a maximum open-fracture intersection per unit length of well requires determining the orientation and distribution of open fractures. Furthermore, a structural model based on regional tectonics is necessary if predictions of open-fracture orientations outside the immediate study area are to be made. The fractures were therefore classified into extension, hybrid and shear fractures, each with their particular relation to the stress field 10, 11. Overprinting relationships observed on the core and in thin section allowed us to differentiate two main phases (Phase I and II) of the fracture/stylolite network development (Fig. 2). Phase I fractures consist of a) wedge-shaped, bed-normal extension fractures associated with bed-parallel stylolites and b) hybrid/shear fractures at large angles to the bedding and bed-normal planar extension fractures (Fig. 2a). In addition to these Phase I fractures, bed-normal extension fractures, partially filled with hydrocarbon residue, occur in the La Luna Formation (Fig. 2b).

METHODS Determining the orientation of fractures is an essential part of an accurate fracture network description. The recovery of the 25/s" diameter cores was, with the exception of a few minor rubble zones, excellent. Paleomagnetic orientation methods were used 7 ,s to orient selected core intervals which contained abundant fractures. In total, 21 core intervals from SVS-225 and 22 intervals from SVS-229, representing respectively some 156 ft (47.5 m) and 40 ft (12.2 m) of core, were selected for paleomagnetic core orientation. Approximately 350 fracture orientations were measured in core SVS-225 and some 80 fractures in core SVS-229. Other fracture parameters (stratigraphic occurrence, density, aperture, etc.) were determined using all available core material recovered from 14915 15430.9 ft (4546 - 4703 m) driller's depth for SVS225 and from 13155 - 13269.6 ft (4010 - 4045 m)

-SPE 23620

Phase II fractures comprise shear fractures at a small angle to the bedding and bed-parallel extension fractures associated with bed-normal stylolites. In addition, bed-normal extension fractures are associated with a later phase of reopening of stylolites (Fig. 2). The fracture sequences are summarized in Tables 1 and 2. Two fracture types are distinguished: cemented (or closed) fractures that will not contribute to production and partially cemented fractures through which fluid can flow. These latter are· partially cemented or

44

SPE 23620

R.C.M.W. FRANSSEN, V.C. VAHRENKAMP, W.J.E. VAN DE GRAAFF AND P.J. MUNOZ

leached fractures with a "channel and island" structure of interconnected porosity (Fig. 3). The invasion of drilling mud in the cores indicates that these highly irregular pore spaces are interconnected and form conduits for fluid flow. The open fractures in the Cogollo Group (SVS-225) are bed-normal extension fractures belonging to Phase lib. These fractures were not observed in the La Luna Formation (SVS-229). In the La Luna Formation, the bed-normal Phase I extenSion fractures, which are partially filled with black heavy hydrocarbons, contain significant porosity. Fracture orientations The orientations of the natural fractures that were observed in cores, as determined by paleomagnetic orientation, are plotted as poles in a lower hemisphere stereographic projection (Fig. 4). In this study, the main emphasis is on the description of the open-fracture network. Therefore, we treat the openfractures as a separate orientation group, although genetically they are part of Phase I (SVS-229) or II (SVS-225). This leads to a subdivision of fracture orientations into three subgroups for the Cogollo Group and four subgroups for the La Luna Formation. The characteristics of these subdivisions are summarized in Tables 3 and 4. The open-fracture network in the Cogollo Group consists of subvertical, ENE-WSW trending extension fractures of Phase II, whereas in the La Luna Formation it consists of the WNW-ESE trending subvertical fractures that were formed during Phase I. The fracture orientation on the West Flank of Block IX (SVS-22S) differs from the East Flank (SVS-229). Firstly, the azimuth of bed-normal Phase I extension fractures in SVS-225 differs by 25 0 - 35 0 from the azimuth of the Phase I fractures observed in SVS-229. This difference is significant given the 50 accuracy of the paleomagnetic orientation technique? . Secondly, the open bed-normal extension fractures with strikes around ENE are not observed in SVS-229 (La Luna Formation). Relation between fracture occurrence and lithofacies DepoSitional cyclicity at several scales was observed both in cores and logs of the shallow water carbonates (Cogollo Group) as well as of the deep marine La Luna Formation (Fig. 5). In the Cogollo Group, open marine grainstones and packstones alternate with mudstones and wackestones of restricted marine origin (the large-scale cycle thickness ranges from 100 to 400 ft). In the La Luna Formation packstones rich in organic material form

3

prominent foot-scale cycles with less organic-rich packstones. . In the Cogollo Group open fractures occur preferentially in the open marinepackstones and grainstones (Fig. 5). The open fractures in the La Luna Formation are contained in the less organic-rich parts of the cycles. The thickness of these fractureprone intervals in the La Luna Formation is below or at the resolution of most conventional logs. Fracture networks and orientation of the stress field Techniques to determine paleostress directions from fracture-orientation data are numerous 12 . However, a detailed stress inversion analysis involves large data sets, ideally from slickensided shear fractures from which the slip direction can be inferred. Since only a limited data set and very few slickensided shear fractures are available, we estimated the paleostress directions from the mean directions of the poles of the fracture orientation groups. These paleostress directions were then verified using shear fractures with unequivocal shear indicators 13 and opening directions of extension fractures (which are assumed to open perpendicular to the minimum principal stress 0"3)' It should be emphasized that the paleostre:;s directions obtained in this way are estimates rather than unique analytical solutions. Subvertical extension fractures and associated reverse shear fractures indicate that, during Phase I, the principal horizontal stress direction (JH in the West Flank was oriented around NNW-SSE. The orientation of Phase I fractures in the East Flank differs by 25 0 -35 0 in a counter-clockwise sense. Thus, the inferred (JH -direction is rotated in a similar manner towards NW-SE. The Phase II fractures in the East and West Flank have similar orientations. The inferred (Jwdirection during Phase II was oriented around ENE-WSW to E-W. We determined the local presentday in-situ (Jwdirection, which is oriented E-W and associated with the Andean compression, using strain relaxation and acoustic transmission techniques and borehole-elongation data. A detailed discussion of these techniques is outside the scope of this paper.

WELL-TRACK ORIENTATIONS FOR OPTIMUM OPEN-FRACTURE INTERCEPTION RATES Method To predict optimum well tracks, a 3-D fracture network 45 was modelled. Parameters which need to be known

4

OPTIMIZING THE FIELD DEVELOPMENT OF DEEP CRETACEOUS FRACTURED ... SPE 23620 RESERVOIRS, LAKE MARACAIBO, VENEZUELA

for modelling purposes are: the orientation distribution, density, length and height of the fractures. Orientation and density have been determined from core measurements. Fracture density has been defined as the number of fractures per unit volume of core. Published methods to estimate true fracture densities 14 imply a systematic relation between fracture spacing and bed thickness - a relation not evident in the massive Cogollo Group limestones. Fracture height and length data could not be obtained from cores, instead, outcrop data or estimates were used. For simplicity the fractures were assumed to be rectangular.

interception rate is obtained till approximately 50° deviation. Any further: increase in deviation angle has a minimal effect on the interception rate. In the case of SVS-229 the maximum open-fracture interception rate is reached for a well azimuth of approximately 30° (Fig. 6a), independent of deviation angle. The rose diagrams are asymmetric because most fractures dip towards SW. As is the case for SVS-225, the open-fracture interception rate for SVS-229 increases up to approximately 60 0 deviation at 30° azimuth (Fig. 7b). Higher deviation angles give only a relatively small increase in the number of fractures intercepted per unit length of well. The total number of fractures intercepted will, of course, increase if the well path is longer for higher deviations.

As a result of our estimation procedure, approximately three times as many fractures were simulated in a unit volume of rock than can be expected in reality. However, the relative amounts of fractures in the different sets are correct and agree with core observations. Subsequently, wells were simulated to penetrate the modelled 3-D fracture network. For any given well orientation the number of fractures intercepted per set was counted and plotted in an interception-rate rose diagram. Simulations were carried out in incremental steps of 10° deviation, from vertical to 90° and with 10° azimuth increments from due North to N100W. In total 325 well tracks were simulated for both the East and the West Flank.

FRACTURE-WIDTH REDUCTION UPON DEPLETION Fluid flow in the tight Cretaceous carbonate reservoirs of Lake Maracaibo is controlled by the open-fracture network. The flow rate of Single-phase, Newtonian, laminar flow in planar fractures is proportional to the cube of the fracture width 11,15. Although the flow through the irregularly shaped, interconnected porosity in the fractures is more complex than plane parallel flow, the impact of the fracture aperture on the flow rate in our simplistic approach still will be very significant16 . Pressure depletion of the reservoir changes the in-situ stress condition, leading to elastic fracture-width reduction, and possibly also to failure of the rock matrix around the fractures.

The modelled fracture network was validated by comparing the pole diagram of the fractures intersected by a simulated vertical well with the pole diagram of the studied core. Our method to determine optimum drilling directions is ·an improvement of the analysis of Nolen-Hoeksema & Howard 2 in the sense that ours honours the statistical orientation distributions of each group of fractures and that the relative fracture denSities of finite fractures is taken into account.

Elastic fracture closure The elastic fracture-width reduction has been calculated from the reservoir pressure reduction, Poisson's ratio, Young's modulus and the geometry of the open fracture (the "channel and island" structure). The initial reservoir pressure for the Deep Cretaceous reservoirs is 0.82 psVft (18.5 kPalm). At present, the reservoir pressure is 0.55 psilft (12.44 kPalm), and the abandonment pressure is 0.26 psilft (5.88 kPalm), which is equal to the bubblepoint pressure. At 13000 ft (3962.4 m) depth, for unbridged channel distances of 0.1 m and initial fracture widths ranging from 1 to 8 mm, the amount of fracture-width reduction ranges from 0.1 to 0.3 mm. Thus, pressure depletion till abandonment leads to a maximum of 10% reduction in fracture width by elastic closure. If the cubic law for fluid flow applies the maximum reduction in flow rate due to elastic fracture-width reduction alone (not taking the

Results The interception-rate rose diagrams for Wells SVS-225 and SVS-229 show the number of open fractures intersected in the shaded part of a petal (Fig. 6). For SVS-225 the highest number of intercepted fractures, irrespective of fracture type, is found for a well azimuth of 230° (Fig. 6b). This maximum is independent of deviation angle. However, the maximum number of open fractures is intersected for an azimuth of 150° or 330°. In order to determine the optimum well deviation the number of open fractures is plotted against deviation angle for 330 0 azimuth (Fig. 7a). A strong increase in the

46

SPE 23620

R.C.M.W. FRANSSEN, V.C. VAHRENKAMP, W.J.E. VAN DE GRMFF AND P.J. MUNOZ Mohr circles will shift away from the failure envelope, but without increasing Significantly in size. The reservoir pressure drops necessary to induce failure around the fractures in impermeable rock exceed the pressure depletion to abandonment, which is 0.29 psi/ft.

pressure depletion into account) is approximately 30%.

Deformation of fracture faces Besides elastic closure, the fracture permeability or conductivity will show additional closure if the rock matrix supporting the channels deforms plastically and/or fails. To determine whether failure will occur as a result of reservoir depletion, we will consider the change of the state of stress at the fracture face for a rock matrix - fracture system in two dimensions.

Application to present situation Three factors suggest that no failure will occur during pressure depletion in the Deep Cretaceous reservoirs: 1) low rock matrix permeability; 2) the orientation of the principal horizontal in-situ stress O"H; and 3) the channel-and-island structure of the fractures.

Permeable rock matrix At the interface between a vertical fracture and the matrix the effective horizontal stress, which is normal to the fracture face, is given by (Fig. 8)

In view of the poor reservoir quality (permeabilities in the JlD range), the state of stress at the fracture face will be close to that in the case of an impermeable rock matrix as described above. The reservoir pressure depletion until abandonment is then not sufficient to induce rock failure. An additional factor reducing the probability of failure is the orientation of open fractures with respect to the present-day in-situ stress field. The vertical open fractures strike subparallel to the maximum present-day horizontal stress direction. The combination of the channeland-island structure of open fractures and loadbearing bridges, high rock matrix strength and the subparallel E-W trend of the present-day maximum horizontal stress indicates that it is unlikely that the conductivity of the individual fractures will be greatly reduced upon pressure depletion.

cr'h= O. The effective vertical stress, which is assumed to be the maximum in-situ stress, is given by

i.e. depletion will increase a'v. Mohr circles representing the state of stress around a vertical fracture are shown for the initial reservoir pressure conditions (0.82 psilft) and for the presentday conditions (0.55 psi/ft). In addition, the MohrCoulomb failure envelope for representative lithologies of the Cogollo group and the La Luna Formation, as determined from triaxial strength tests, are plotted (Fig. 8). From this analysis it is clear that failure as a result of depletion will not occur in the Cogollo Group limestones. In the La Luna Formation, on the other hand, failure around fractures should have occurred if the rock matrix was permeable.

REGIONAL VALIDITY OF THE FRACTURE NETWORK MODEL Sedimentological, diagenetic and structural characteristics of the Deep Cretaceous carbonate reservoirs in Block IX are consistent with data obtained from Wells VLA-711 and VLA-722 in the West Flank of Block I, some 40 km to the north 17 (Fig. 1). The cyclicity of open marine depoSits alternating with deposits of restricted marine origin as observed in the Cogollo Group (Fig. 5) can be correlated with Block I data; the cyclicity concept is very useful for basin-wide correlations. Fractures occur preferentially in the grainstones and packstones of the open marine cycles. Since the open marine cycles can be recognized from logs (Fig. 5), the fracture-prone intervals can be recognized indirectly by their log Signature.

Impermeable rock matrix If the rock matrix is impermeable, then a different set of relations apply. The effective horizontal stress is then equal to the pore-fluid pressure in the fracture,

and the effective vertical stress is then equal to the overburden weight, ,

cr v= crv· The effective stress values are thus higher; however, the stress differences (the radii of the Mohr Circles) are similar to those for a permeable matrix. In other words, in the case of an impermeable matrix, the

47

The geometry of the fracture network observed in cores from Wells SVS-225 resembles the fracture network observed in cores from the West Flank in Block P7. No previous data from the East Flank of Block I are available. Phase la of the fracture system is

6

OPTIMIZING THE FIELD DEVELOPMENT OF DEEP CRETACEOUS FRACTURED RESERVOIRS, LAKE MARACAIBO, VENEZUELA

1200 on the East Flank and 060 0 on the West Flank. The presence and orientation of open fractures may depend on the position of the well, relative to the Icotea fault, and on lithology. A fourfold increase in the fracture-interception rate, relative to a vertical well, is obtained with well tracks towards azimuth 330 0 and deviation 60 0 in the West Flank and towards azimuth 0300 and deviation 60 0 in the East Flank.

associated with initial compaction during burial. Subsequent development of the fracture network can be interpreted in terms of the kinematic evolution of the region. Fracturing Phases Ib,c are related to the Icotea strike-slip faulting (NW-SE compression from early Eocene until late Oligocene3 ). Phase II fractures are related to the Andean orogeny with an E-W compressional regime, from late Miocene times to the present-day4,5. 2.

The "channel and island" structure of the open fractures, the high rock matrix strength and the subparallel fracture azimuth relative to the E-W trending principal stress direction all contribute to the conclusion that the individual fracture conductivity is not greatly reduced upon depletion.

3.

Depositional cyclicity at several scales has been observed in both cores and logs of the shallowwater limestones of the Cogollo Group as well as of the deep marine La Luna Formation. In the Cogollo Group intervals composed of open marine grainstones and packstones and intervals dominated by mudstones and wackestones of restricted marine origin form large-scale depositional cycles (cycle thickness: 100 to 400 tt). Open fractures occur predominantly in the open marine cycle halves. In the La Luna Formation organic-rich limestones alternate with limestones poor in organic matter (cycle thickness: 1 to 3 ft). Open fractures are contained in the less organic-rich intervals.

4.

The reservOir geological model established is consistent with data obtained from wells in the West Flank of Block I, some 40 km to the north. The well track recommendation of azimuth 3300 should also apply to the West Flank of Block I.

LIMITATIONS The detailed geometry of the fracture networks is not only described by the orientation of the fractures but also by their dimensions, interconnectedness and areal distribution. The orientation of fractures depends not only on the orientation of the far-field stress but also on the mechanical response of the rock (e.g., composition, bedding thickness, diagenetic signature, etc.), on the proximity to larger-scale faults (structural control) and on the exact orientation of the local stress field in the neighbourhood of large-scale faults. Fracture interconnectedness depends not only on the orientation distribution of the open fractures but also on their aspect ratios and length distributions. These factors are unknown in the present case. Fracture length and height were assumed to be uniformly distributed, but this is not the case in nature 18 . Finally, fault proximity cannot be used as an indicator for the areal distributions of fractures, because the fault poSitions and orientations are poorly known and the relationship between fracture density and fault proximity is not known for the Deep Cretaceous reservoirs.

SPE 23620

Although in our computer models for both SVS-22S and SVS-229, more open fractures are intersected per unit length of simulated core than are observed in reality, the distribution of the fracture orientations and their relative amounts correspond with those inferred from cores. Hence, the probabilistic modelling approach is suitable for optimizing welltrack orientations. Since the detailed geometry of the fracture network is not fully described by core observations only, the derived probabilistic models are not suitable as input for reservoir simulations or for STOIIP calculations.

NOMENCLATURE

CONCLUSIONS 1 . The production of hydrocarbons in the Deep Cretaceous carbonate reservoirs in Lake Maracaibo is controlled by open fractures. The open fractures are subvertical and have a strike of

48

0"3

=

O"H

=

O"'h O"v

= =

o"'v Pf

=

minimum principal stress component maximum stress direction in the horizontal plane minimum effective horizontal stress total vertical stress effective vertical stress pore fluid pressure

SPE 23620

R.C.M.W. FRANSSEN, V.C. VAHRENKAMP, W.J.E. VAN DE GRAAFF AND P.J. MUNOZ

ACKNOWLEDGEMENTS The authors acknowledge the contribution of E.J.M. Willemse, R. Lagazzi and C.G.L. Mercadier (KSEPL) to this study and their fracture description of Block I core data. P.J. van den Hoek performed the rock mechanical tests and analysis. Valuable discussions with R. Lagazzi, F. Chacartegui, E. Bueno and Z. Sancevic, all of Maraven S.A., are gratefully acknowledged. Finally, Shellintemationale Research Maatschappij BV and Maraven SA are thanked for permission to publish this paper.

2.

3.

Vahrenkamp, V.C., Franssen, R.C.M.W., van de Graaff, W.J.E. and MuFloz, P.J. : "The Correlation of a Multi-Level Porosity System with Depositional Cyclicity and Log Signatures: Deep Cretaceous reservoirs, Lake Maracaibo, Venezuela, " AAPG Bull. 75 (1991) 686-687.

5.

Pennington, W.D.: "Subduction of the Eastern Panama Basin and Seismotectonics of Northwestern South America," J. Geophys. Res. 86 (1981) 10753-10770.

6.

Gonzales de Juana, C., Itturraide de Aranzona, J., and Picard, X.: Geologia de Venezuela y sus Cuencas Petroliferas, Foninves (1980) 1030.

7.

Bleakly, D.C., Van Alstine, D.A. and Packer, D.A.: "How to Evaluate Orientation Data, Quality Control," Oil and Gas Journal (Dec. 1985) 46-52.

Kulander, B.A., Dean. S.L., and Ward, B.J.: Fractured Core Analysis: Interpretation, Logging, and Use of Natural and Induced fractures in Core, AAPG, Tulsa (1990), 88.

12. Angelier, J.: "Tectonic Analysis of Fault Slip Data Sets," J. Geophys. Res. 89 (1984) 5835-5848. 13. Petit, J.P.: ·Criteria for the Sense of Movement on Fault Surfaces in Brittle Rocks," J. Struct. Geol.9 (1987) 597-608.

Kellog, J.N.: "Cenozoic TectoniC History of the Sierra de Perija, Venezuela-Colombia," The Caribbean-South American plate boundary and regional tectonics W.E. Bonini, A.B. Hargraves, and R. Shagam (eds.), Geol. Soc. Am. Mem. 162 (1984) 239-261. Ross, M.I. and Scotese, C.R.: "A Hierarchical Model of the Gulf of Mexico and the Caribbean Region," Tectonophysics 155 (1987) 139-168.

9.

11. Nelson, A.A.: Geologic Analysis of Naturally Fractured Reservoirs, Gulf Publishing Company, Houston (1985) 320.

Nolen-Hoeksema, R.C. and Howard, J.H.: "Estimating Drilling Direction for Optimum Production in a Fractured Reservoir," AAPG Bull. 71 (1987) 958-966.

4.

Van Alstine, D.A., Bl(tterworth, J.E., Willemse, E.J.M. and van de Graaff, W.J.E.: Paleomagnetic Core-Orientation for Characterizing Reservoir Anisotropy: Case Histories from Fractured Reservoirs in Abu Dhabi and Venezuela," AAPG Bull. 75 (1991) 687.

10. Price, N.J. and Cosgrove, J.W.: Analysis of Geological Structures, Cambridge University Pres, Cambridge (1990) 502.

REFERENCES 1.

8.

7

14. Narr, W. and Lerche, I.: " A Method for Estimating Subsurface Fracture Density in Core," AAPG Bull. 68 (1984) 637-648. 15. Witherspoon, P.A., Wang, J.S.Y., Iwai, K., and Gale, J.E.: "Validity of Cubic Law for Fluid Flow in a Deformable Rock Fracture," Water Resources Res. 16 (1980) 1016-1024. 16. Cook, A.M., Myer, L.A., Cook, N.G.W., and Doyle, F.M.: "The Effects of Tortuosity on Flow through a Natural Fracture," in: Rock Mechanics Contributions and Challenges, Balkema, Rotterdam (1990) 371-378. 17. Willemse, E.J.M., van de Graaff, W.J.E. and Sancevic, Z.: "Characterization of an Overpressured, Cretaceous Carbonate Reservoir, Lake Maracaibo, Venezuela," AAPG Bull. 74 (1990) 791. 18. Kulatilake, P.H.S.W.: "State-of-the-Art in Stochastic Joint Geometry Modelling," Proc. 29 th US Symposium on Rock Mechanics, Minneapolis (1988) 215-229.

49

8

OPTIMIZING THE FIELD DEVELOPMENT OF DEEP CRETACEOUS FRACTURED RESERVOIRS, LAKE MARACAIBO, VENEZUELA

·SPE 23620

Table 1: Sequence of fracture events In cores from well SVS-225 (Cogollo Group) Phase la-

bed-parallel stylolites and associated wedge-shaped extension fractures normal to bedding hybricJIshear fractures and 'rod-shaped' extension fractures, all normal to bedding bed-parallel fractures (shear/hybrid/extension) and bed-normal stylOlites. bed-normal hybricJIextension fractures with local porosity in the fracture plane and reopened stylolites bed-normal shear fractures with strike-slip components.

Phase IbPhase lIaPhase IIbPhase I-II

Table 2: Sequence of fracture events in cores from well SVS-229 (La Luna Formation) Phase la-

bed-parallel stylolites and associated bed-normal. wedge-shaped fractures hybricJIshear and extension fractures normal to bedding (calcite cemented). bed-normal extension fractures partially filled with black heavy hydrocarbons containing local porosity in the fracture plane. bed-parallel shear and extension fractures and bed-normal stylolites. bed-normal strike-slip shear fractures.

Phase IbPhase IcPhase lIaPhase I-II

Table 3:

Fracture orientation groups for SVS-225: Cogollo Group

Dip

Strike

Type

Subvertical Subhorizontal

NW-SE slight preference for E-W ENE-WSW

extension/hybrid (cemented) lab shear with reverse (not open) lIa dip slip extension/hybrid (open) lib

Subvertical

Table 4:

Phase

- West Flank Compression direction NW-SE E-W E-W

Fracture orientation groups for SVS-229: La Luna Formation - East Flank

Dip

Strike

Type

Phase

Compression direction

Subvertical Subvertical Subhorizontal Subhorizontal

NW-SE WNW-ESE WNW-ESE slight preference for NE-SW

extension (cemented) extension (open) extension (cemented) shear with reverse dip slip (not open)

lab Ic lIa lIa

NW-SE WNW-ESE E-W E-W

50

;---. VLA-711 I

_

1

j1

________ 1

Ii

5km

nI iii 0:1 ... .20.

S1

0

1

: :

1 1

: It

Icote~ 1

()

01 0 0'"

at!)

La Luna Maraca Lisure Apon

1-.-1-.-

fault

'1

"V~~~

lIa

ISVS - 229

lIa

b5 ... SVS - 225

'-' '"' ~ ~"~ Q~nt;; '-' ~~

lIa lIa

h-l-T

Rio Nell."o

: Block IX

1.. _ _ _ _ _ _ J

Fig. 1

....

f:

1

L--J

VI

1

lIa Ib

foLr

Socuv

1

N o

:J 0

{f] '-,

SVS-229 SVS;225

til

I-II Ib

-

Colon

Ql

I-.

lIa - " Ib Reopened Ie

- -- -- --- --Cored - - - - Intervals - - - -- - -

VI

I 1

1

j~

Block I

'-' '-'

Orgamc rich -lia

~~ f\\~

1\\+

Pre-Cretaceous "Basement"

SVS - 225 Cog 011 a Group

1+++

Location of study wells SVS-225 and SVS-229 in Block IX, Lake Maracaibo, Venezuela

Fig. 2

SVS - 229 La Luna Formation

Fracture/Stylolite overprinting relations as observed on cores wells SVS-225 and SVS-229

N

N