Depositional Themes of Mixed Carbonate ... - ETH Limnogeology

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McNeill, D. F., K. J. Cunningham, L. A. Guertin, and F. S. Anselmetti, 2004, Depositional themes of mixed carbonate-siliciclastics in the south Florida Neogene: Application to ancient deposits, in Integration of outcrop and modern analogs in reservoir modeling: AAPG Memoir 80, p. 23 – 43.

Depositional Themes of Mixed Carbonate-siliciclastics in the South Florida Neogene: Application to Ancient Deposits D. F. McNeill Comparative Sedimentology Laboratory, University of Miami, Miami, Florida, U.S.A.

K. J. Cunningham1 Comparative Sedimentology Laboratory, University of Miami, Miami, Florida, U.S.A.

L. A. Guertin Pennsylvania State University, Media, Pennsylvania, U.S.A.

F. S. Anselmetti Geological Institute, ETH, Zurich, Switzerland

ABSTRACT

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recent drilling project to evaluate the Neogene stratigraphy of south Florida has provided refined insight to the depositional controls and facies patterns of a heterogeneous, mixed carbonate-siliciclastic system. Six key themes have emerged that may have implication for reservoir development and facies architecture in similar depositional systems. These ‘‘modern’’ depositional themes are compared to some ancient mixed system examples. Although mixed systems are complex and spatially unique, similarities in the basic lithofacies deposition and their associated physical properties can aid in prediction of reservoir distribution and in refinement of geologic models in ancient mixed systems. The deposition-related themes recognized in this study of the Florida Neogene include (1) Concept of Template Control on Both Carbonate and Siliciclastic Deposition — precursor topography controls depositional geometry and location of subsequent depocenters for both carbonates and siliciclastics; (2) Distal Transport of Coarse Clastics and Influence of Currents on Grain-size Segregation— conditions can exist for the long-distance transport (fluvial?) of extremely coarse siliciclastics (flatpebble quartz in this Neogene example) from the source area, and regional currents help segregate grain-size populations and partition grain types; (3) Demise of the Carbonate 1

Current affiliation: U.S. Geological Survey, Miami, Florida, U.S.A.

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Platform/Ramp: Smothered by Siliciclastics?— in this Neogene example, we recognize a hiatus of several million years bounding the top of a carbonate ramp, which indicates that demise of the ramp and subsequent input of siliciclastics are temporally distinct; (4) The Mixing Transition: Abrupt Vertical and Lateral Facies Changes— the lateral transition of carbonate to siliciclastic strata highlights the potential for abrupt facies changes both laterally and vertically. Interfingered carbonates and siliciclastics may form stratigraphic traps based on lithologic differences and differential diagenesis and can result in alternating reservoir pay zones and nonreservoir intervals; (5) Cryptic Sequence Boundary in Shallow-marine Siliciclastics and Carbonates— in cases where no distinct change in lithology exists, it may be inherently difficult to recognize major disconformity based only on lithologic changes. In settings dominated by admixing, sequence-boundary confirmation may require the integration of biostratigraphic and chemostratigraphic markers with any available textural indicators; and (6) Similarity in Acoustic Properties of Laterally Equivalent Siliciclastics and Carbonates— shallow burial and early diagenesis have produced an almost identical acoustic signature for the two admixed sediment types. This acoustic similarity may make it difficult to distinguish specific lithofacies on seismic profiles and sonic logs. In ancient mixed-system deposits where only seismic data exist, problems in specific lithofacies or geometric characterization may occur.

INTRODUCTION To better understand the subsurface geology of the southern Florida peninsula, the University of Miami and Florida Geological Survey collaborated in the mid1990s on the collection of a series of continuously cored borings in the Florida Keys and southern mainland of Florida (Figure 1A). This drilling project was informally termed the Florida Keys Drilling Project (FKDP). The FKDP had three major objectives: (1) to better characterize the sediments and facies that comprise a relatively young mixed carbonate-siliciclastic sedimentary system; (2) to elucidate the complex and heterogeneous stratigraphic relations inherent in this system; and (3) to provide a baseline case study on the physical factors that influence the spatial distribution of mixed carbonate-siliciclastic lithofacies and thus identify characteristics of potential reservoir facies. These results form the basis for a case study applicable to ancient mixed systems. To that end, this paper distills recently published results of the drilling and subsequent analyses to compile six deposition-related ‘‘themes.’’ The identification of these themes is based on the successful integration of data from cores, cuttings, geophysical logs, and marine-seismic profiles to provide new stratigraphic, petrographic, chronologic, petrophysical, and diagenetic information on these Neogene mixedsystem sediments and their distribution. The key publications from the FKDP include Warzeski et al. (1996), Anselmetti et al. (1997), Cunningham et al. (1998), and Guertin et al. (1999, 2000). The six depositional themes have relevance to the exploration and development of oil and gas reservoirs

in ancient mixed systems. The FKDP core-based stratigraphy, seismic data, and a regional compilation of existing cuttings and geophysical log data have provided an age-constrained database for the establishment of temporal and spatial relations between carbonate and siliciclastic sediments. Data from this Neogene example can be extrapolated to deep burial conditions, but the physical changes (and range of variation) that result from burial diagenesis should be considered. These depositional themes are valuable in that the original textural lithofacies often directly influences the type and degree of subsequent diagenesis. This combination of original texture and burial diagenesis is a major control of porosity and permeability development in ancient rocks. Fortunately, these facies relationships and their influence on reservoir potential are commonly predictable with a basic knowledge of sediment source area, the mode of sediment input, mixing dynamics, and paleogeography. Results from this project serve to provide a case study helpful to the understanding, conceptualization, and interpretation of ancient mixed-system lithofacies and their variability.

REGIONAL GEOLOGIC SETTING OF THE MIXED SYSTEM The late Neogene mixed carbonate-siliciclastic system in south Florida includes two types of mixing (sensu Budd and Harris, 1990). In vertical replacement mixing, limestones and siliciclastics replace one another and often result in alternating beds of each end-member

Depositional Themes of Mixed Carbonate-siliciclastics in the South Florida Neogene

FIGURE 1. (A) A general location map of the five core borings collected as part of the Florida Keys Drilling Project. Some drill sites in the western Bahamas are also shown. (B) Revised stratigraphy of the subsurface of the Florida Keys and southernmost peninsula following that proposed by Cunningham et al. (1998) and Guertin et al. (2000). In the Florida Keys, the Peace River Formation is absent. Cunningham et al. (2001a) have recently revised the Neogene stratigraphy of much of the southern peninsula of Florida, especially the age of deposits in the Peace River Formation. The Long Key and Stock Island Formations are the primary siliciclastic and carbonate units, respectively, discussed in this paper. (C) Geologic cross section along the study area from the southwest (A) to the northeast (A0). Note the lateral mixing of the Stock Island and Long Key Formations. The figure in (C) is modified from Cunningham et al. (1998) and used with permission of the Geological Society of America.

lithology. In lateral mixing, contemporaneous siliciclastic and carbonate facies admix because of variability in siliciclastic supply and/or lateral facies shifts. The key geologic units in this study have been dated using a combination of strontium-isotope stratigraphy (carbonates) and planktonic-foraminiferal biostratigraphy (siliciclastics) (Figure 2) (Guertin, 1998; Guertin et al., 1999). These refined ages constrain the temporal nature of mixed deposition with respect to major discontinuities and proposed sea level changes (Haq et al., 1987, 1988). Four formational units are included in the mixed system. Starting from the base, they include, first, the lower to middle Miocene Arcadia Formation, a carbonate ramp that is a composite sequence composed of four high-frequency sequences (Cunningham et al., 1998). The principal grains of the carbonate ramp are skeletal fragments of mollusks, benthic foraminifera, red algae, and echinoids, an assemblage of grain types consistent with production in temperate water ( James, 1997). Although predominantly carbonate, the Arcadia Forma-

tion can contain as much as several percent quartz sand and phosphorite grains. The abundance of these noncarbonate components increases from the Florida Keys northward across the peninsula. The top of the Arcadia Formation is a regional unconformity that is replaced by a thin, black phosphorite layer. The second formational unit is an upper Miocene to upper Pliocene mixed carbonate siliciclastic unit that is predominantly siliciclastic in the northeast (Long Key Formation, as much as 145 m thick) and carbonate in the southwest (Stock Island Formation, 120 m thick). The newly defined Long Key Formation (Cunningham et al., 1998) is a quartz-sand unit with varying amounts (500 km) transport of pebbles and small cobbles via fluvial processes. They describe middle Cretaceous coarse siliciclastics that were deposited in incised valleys cut in Paleozoic strata of Iowa and eastern Nebraska, along the eastern margin of the Western Interior Seaway. Brenner et al. (2001) have speculated that long-distance transport was related to seasonal monsoon climatic conditions that characterized the region during the middle Cretaceous. A second example of pebble and cobble transport (of at least 100 km) is the example described from the Albian mixed systems of northern Spain by Garcıá-Monde´jar and Fernańdez-Mendiola (1993). They describe quartzite and sandstone pebbles that followed a paleokarst limestone surface and likely were transported into the predominantly carbonate setting by strong fluvial currents during subaerial exposure. Garcıá-Monde´jar and Fernańdez-Mendiola (1993) suggest that the fines were

selectively removed from the siliciclastics during the subsequent transgression, a partitioning scenario very similar to that proposed in our south Florida example. The distal transport of fine and medium sand-sized material and both horizontal and vertical mixing with carbonate sediments is a relatively common process, and numerous examples (i.e., the Permian Basin) exist. Although the input of gravel-sized material is less commonly reported, we propose that it can be a significant component of mixed carbonate-siliciclastic depositional systems when an adequate source exists. The demise of carbonate platforms by the influx of siliciclastics (lowstand burial excluded) still is a widely debated topic. From a sequence-stratigraphic viewpoint, Schlager (1989, p. 17) argued that ‘‘For the final geometry, it makes little difference whether a platform was killed by rapid submergence and later buried by siliciclastics, or whether burial by the siliciclastics caused the demise.’’ From a geologic standpoint, however, the timing of siliciclastic input relative to platform demise is important for both regional depositional models as well as reservoir models and seal prediction. An


gin and shelf deposits (e.g., Cambrian of central Texas, King and Chafetz, 1983; Upper Permian of the Permian Basin, Mazzullo et al., 1991; Upper Cretaceous of Angola, Lomando and Walker, 1991). One classic example is the Yates Formation in the Central Basin Platform in the Permian Basin (Borer and Harris, 1989, 1991a, b; Mutti and Simo, 1993), and others exist in the same basin (e.g. the Queen and Grayburg Formations). In the Yates Formation example of Borer and Harris (1991b), a series of cores along a dip-oriented transect documents the lateral mixing of siliciclastics and carbonates at a middleshelf location behind the carbonate-rich shelf margin. The main zone of mixed sediment on the middle shelf was about 25 – 30 km wide. The cyclic nature of the siliciclastic and carbonate beds on the middle shelf, in conjunction with biostratigraphic age control, led to the interpretation that low-amplitude sea level changes caused by FIGURE 15. Sonic-velocity logs and discrete-sample velocity data from the Stock orbital forcing were the main conIsland and Long Key Formations. The carbonates of the Stock Island Formation trol on sediment type. If interfinand the siliciclastics of the Long Key Formation have remarkably similar acoustic gered carbonates and siliciclastics characteristics. This similarity makes the seismic determination of lithofacies extremely difficult using seismic data alone. Several ancient examples discussed of the Yates Formation were a result of high-frequency sea level in the text show that siliciclastic and carbonate admixtures sometimes retain this acoustic similarity through burial. The figure is from Anselmetti et al. (1997) and changes, these alternating lithologies appear also to have conused with permission of Elsevier Publishing. trolled the eventual reservoir properties (Borer and Harris, 1991b). In interpretation that commonly is proposed for a sharp the Yates, siliciclastic sand (lowstand deposit) forms the boundary in which siliciclastics overlie carbonate is one main reservoir units and is interbedded with nonreserof decreased or terminated carbonate production caused voir carbonates (highstand deposit). In the lowstand by smothering or burial. The Florida Neogene example siliciclastics, spatial differences exist in reservoir quality adds to the growing evidence that not all deepeningbecause of local controls such as the steeper depositional upward, carbonate-to-siliciclastic sequences are a result slope, locally faster subsidence, and the amount of winof a burial smothering mechanism. The approximately nowing related to current energy. We might expect some 8-m.y. hiatus between the final carbonate deposition of the same controls to have produced the interfingered and the first siliciclastic accumulation clearly points to carbonate-siliciclastic facies of the Florida example. Evensome other cause of platform demise. An excellent tual reservoir properties likely will be determined by example of a carbonate platform subjected to environsimilar depositional influences: high-frequency sea levmental stress prior to siliciclastic burial and the occurel changes, sediment grain-size partitioning by currents, rence of a considerable hiatus between the carbonate and progressive differences in diagenesis and cemenand siliciclastic deposition was described by Erlich et al. tation between adjacent beds. These influences might (1990, 1993) for the Upper Jurassic – Lower Cretaceous ultimately produce stacked reservoir and nonreservoir carbonate platform of the Baltimore Canyon area. intervals, as found in the Yates Formation example. The horizontal mixing and stratigraphic interfinThe acoustic characteristics of mixed carbonategering of carbonates and siliciclastics described in the siliciclastic sediments were found to be nearly identical Florida example is similar to many ancient platform mar(low reflectivity, nearly transparent acoustically) in

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the Florida example. Thus, it was difficult to image the anatomy of horizontally mixed and interfingered facies using seismic data alone. In most cases, however, we would expect burial diagenesis to produce some contrast in acoustic signature of the interbedded limestones and sandstones. Where the two sediment types are admixed on the shelf margin or ramp setting, the acoustic signature also can be nearly transparent. Several key examples exist for shelf-margin deposits similar to those in the Florida Neogene. Southgate et al. (1993) have documented a Late Devonian – Mississippian mixed carbonate-siliciclastic ramp system in which highstand deposits lack internal reflection. These acoustically transparent platform deposits apparently make recognition of seismic-based lithofacies or geometries difficult in the highstand deposits. Fortunately, cuttings and cores were available to document the admixed and interbedded nature of the sandstone, mixed sand dolostone, and dolostone lithofacies on the inner ramp (Southgate et al., 1993). In another interesting example, Meyer (1989) discusses the influence of siliciclastics on the Mesozoic platform of the Baltimore Canyon trough. In the shelf-margin system (Kimmeridgian–Berriasian), he describes admixing and interbedding of quartzose sand and silt with shallow-water carbonate grainstone and framestone textures. This shelf-margin mixed system has velocity logs that correlate to a seismically quiet zone with poor reflection characteristics. Acoustic reflectivity improves substantially outward from the shelf-margin system into both the adjacent slope and shallow-shelf deposits where admixed carbonates and siliciclastics are less prevalent. In cases for which only seismic data exist, the low reflectivity or transparent character may even be a useful tool in identifying and mapping mixed deposits.

SUMMARY OF COMPARATIVE THEMES Six depositional themes for a late Neogene mixed carbonate-siliciclastic system have been identified to help understand facies relations of ancient carbonatesiliciclastic deposits. An improved understanding of the factors that influence mixing is necessary to generate accurate regional geologic models and provide some predictability to these spatially and temporally complex facies. Although each mixed system is unique with respect to facies geometry, spatial scale, and sediment input, once understood, the relationship between carbonates and siliciclastics becomes considerably less intimidating. These fundamental themes can be applied at different scales. At the regional geology scale, and with limited two-dimensional seismic data and only sparse well and log data, the explorationist should be aware of

relatively abrupt, lateral lithofacies changes that can occur in mixed carbonate-siliciclastics settings. That these lithofacies changes may not always be distinguishable in seismic data is especially important. With progressive burial, the hope is that diagenesis will develop sufficient petrophysical contrasts to distinguish the main lithofacies in log and seismic data. Similarly, at the regional scale, the geophysical log signature of mixed siliciclastics-carbonates can be nonunique, making accurate lithofacies interpretations difficult when cuttings are unavailable. In a more positive slant, the predictive capability of the petroleum geologist can be enhanced significantly with conceptualization of a depositional template (either erosional or structural). It has long been realized that reefs beget reefs, but probably just as important is that limestone paleotopography can have important control on siliciclastic distribution and facies, and conversely, that siliciclastics can provide a foundation for carbonate initiation and development where none existed previously. Thus, where feasible, geologic modeling should evaluate and try to incorporate the effects of a paleotopographic template in the mapping of reservoir facies. At the more local and development scale, the longstanding (and largely incorrect) dogma that siliciclastics limit carbonate deposition has been challenged with numerous recent examples of reef-siliciclastic coexistence. As such, the nature of a vertical lithofacies change from carbonate to siliciclastic needs to be cautiously interpreted with respect to cause and effect in regional geologic models. It should also be recognized that coarse siliciclastics (gravel as well as sand-sized sediment) can be transported long distances into predominantly carbonate settings. More importantly, local processes at the depocenter (in this case, the interaction of fluvial deltaic and ocean-current processes) can act to sort the transported material by grain size and hydraulic equivalence. In the south Florida example, fluvial and ocean-current processes concentrated and segregated the predominantly coarse and fine siliciclastic sediment. This hydraulic sorting and partitioning can form reservoir facies with compartmentalized properties, especially porosity and permeability. The south Florida example serves to illustrate the importance of evaluating local and regional ocean currents in coastal basins as part of the overall basin analysis model. These current-related processes may be just as important as eustatic sea level changes in sculpting the local sequence geometry. Last, the mixing transition for laterally equivalent carbonates and siliciclastics may be important in generating subtle stratigraphic traps as burial progresses and different diagenetic facies evolve. In this case, the mixing transition appears to be abrupt at the regional scale, although in reality, the interfingering of limestone and siliciclastic facies over a 40-km width would provide both the thickness and


spatial dimension for subtle stratigraphic traps. Similarly, stacked reservoir and nonreservoir units may result from the horizontal and vertical mixing at the carbonatesiliciclastic transition.

ACKNOWLEDGMENTS This drilling project was made possible through assistance of the Florida Geological Survey. Their skilled team provided exceptional recovery of cores at all the drill sites. We appreciate the efforts of Tom Scott and Ken Campbell, who managed the survey drilling program. Robert Ginsburg’s longstanding efforts and interest in the siliciclastics beneath south Florida were the impetus for the (long overdue) drilling. The project also benefited greatly from the initial work of Robert Warzeski. Financial support of the project came from the donors of the American Chemical Society — the Petroleum Research Fund and the industrial associates (ChevronTexaco, ConocoPhillips, Total, ExxonMobil, Japan National Oil, Encana, Shell, and Statoil ASA) of the Comparative Sedimentology Laboratory, University of Miami. The thoughtful, constructive reviews of Gene Rankey, Sal Mazzullo, and Mitch Harris are greatly appreciated.

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