Abstract Tengiz Field is a steep-sided, isolated carbonate platform in the Precaspian Basin, Kazakhstan, with hydrocarbon production from Carboniferous platform and slope facies. Systematic differences in reservoir pressure decline during production indicate that this reservoir consists of three subcompartments or material balance regions: (1) a central “platform reservoir” made up of cyclic platform-top facies that acts like a single, stratified, multistory reservoir; (2) a “wedge reservoir” formed by a prograding margin containing upper slope microbial facies; and (3) an “apron reservoir” containing allochthonous facies deposited in deep water around the base of the buildup. The facies in the apron reservoir accumulated during an early depositional stage and were subsequently partly to fully buried by prograding microbial slope facies of the wedge reservoir. The wedge and apron reservoirs together form a succession 800 to 1000 m thick within the Tengiz oil column. The wedge reservoir shows uniform pressure decline with time and is well connected. Field data (cores and well logs) are insufficient to determine internal continuity of lithofacies and depositional environments or to quantify the pore network responsible for the high connectivity. An outcrop analog (Asturias, Spain) with facies matching those observed in Tengiz cores was used to predict that the microbial lithofacies form a distinct and continuous mechanical unit within the wedge reservoir. Tengiz microbial facies contain a high concentration of solution-enlarged, syndepositional and other early fractures oriented parallel and normal to depositional strike. Borehole image logs provide data on enlarged fracture apertures and local fracture density, but no data related to fracture height or length. An outcrop analog with early fractures in similar facies (Windjana Gorge, Australia) was used to obtain large-scale height and spacing data for solution-enlarged syndepositional fractures. Dissolution processes in the outcrop are different from those of Tengiz, but the fracture aperture and cavern sizes are comparable to their known counterparts in the Tengiz wedge reservoir, and application of the outcrop height data to geologic models of the Tengiz wedge subcompartment can account for its dynamic behavior. The apron reservoir shows a nonsystematic pressure decline with time and is less depleted than the wedge reservoir. The irregular decline indicates reduced internal connectivity within the apron reservoir, which is corroborated by core and borehole image data indicating high lithofacies heterogeneity and the absence of continuous microbial facies responsible for reservoir continuity in the wedge reservoir. A reservoir pressure increase of 1700 psi from the wedge reservoir to the apron reservoir observed in a single well penetration suggests reservoir communication between them may be reduced across a stratigraphic baffle. The wedge and apron reservoirs both contain a late burial matrix diagenetic overprint represented mainly by co-precipitated bitumen and calcite cement and local development of matrix microporosity. Enlargement of the early fractures in the wedge reservoir also occurred during burial diagenesis based on the presence of diagenetic halos containing the burial overprint around the fractures and based on the presence of co-precipitated bitumen and calcite in the fractures. Scenarios and mechanisms for fracture enlargement are evaluated against the observations from field data and the outcrop analogs.

Abstract The well-exposed Djebel Bou Dahar (DBD) carbonate platform (Lower Jurassic, High Atlas, Morocco) demonstrates the role played by different carbonate factories on the growth and architectural evolution of a high-relief, flat-topped carbonate depositional system. It also shows, in contrast with the generally accepted idea that lithiotid bivalve accumulations dominated Lower Jurassic platform margins, that microbial carbonates substantially contributed to the carbonate factory, as in Upper Jurassic reefs. The DBD carbonate depositional system accumulated on the footwall high of an active marine rift. Its depositional architecture evolved from a low- relief ramp profile (Hettangian p.p.-Sinemurian) to a high-relief platform with slopes up to 32° and 600 m in relief (uppermost Sinemurian- Pliensbachian) as a function of changes in accommodation and carbonate factory. The Sinemurian low-relief system consisted of siliceous sponge microbial mounds associated with coated grain skeletal packstone and grainstone in middle and outer ramp facies belts. This deep-water carbonate factory did not build into wave-agitated shallow settings and lacked the capability of constructing high-relief platform margins. From the latest Sinemurian, the platform built significant relief and the slope steepened (20-32°). This switch in platform architecture coincided with the accumulation of a highly productive, coral calcareous sponge microbial boundstone at the platform margin and on the slope (from 10 to 60 m in depth). This was adjacent to deeper water siliceous sponge microbial boundstone (from 60 to 140 m below the platform break). During the late Pliensbachian increased accommodation and retrogradation, coral calcareous sponge microbial boundstone extended from the upper slope onto the outermost platform, 350 to 400 m inward of the platform break, associated with microbial microencruster boundstone and lithiotid bivalve biostromes. During this aggradational- retrogradational phase, microbialites were able to expand on the outer platform top because low-energy substrates were made available on the platform top by increased accommodation. Outer platform strata consisted of coral calcareous sponge microbial boundstone and coated grain skeletal grainstone, dipping 5° basinward, as observed in other Mesozoic and Paleozoic microbial boundstone-dominated platform margins. The platform interior was dominated by subtidal peloidal skeletal packstone with Cayeuxia-calcified cyanobacteria and intertidal fenestral packstone with laminated microbial boundstone, which contributed to the sediment budget maintaining a flat-topped platform interior geometry. The DBD shares similarities for facies and depositional geometry with Upper Jurassic Southern Tethyan and North Atlantic carbonate systems, implying that the main components of Upper Jurassic reefs were already present in the Early Jurassic rift basin of Morocco.

Abstract Among the more challenging questions in geology are those concerning the anatomy of sedimentary bodies and related stratal surfaces. Though significant progress has been made on the interpretation of depositional environments, little systematic data are available on their dimensions and geometry. With the recent advances in computer power, software development and accuracy of affordable positioning equipment, it has now become possible to extract highresolution quantitative data on the anatomy of sedimentary bodies. In Asturias, northwestern Spain, aerial photography provides continuous 2-D cross-sections of a seismic-scale, rotated to vertical, carbonate platform margin of the Early Carboniferous. Digital elevation models, orthorectified aerial photographic imagery and ground verification of stratal surfaces generated the elements that are required to reconstruct the true dimensions, angular relationships of bedding planes and the spatial distribution of facies units in this platform margin. Along with biostratigraphy this provides sufficient constraints to estimate rates of progradation, aggradation, growth and removal of sediments in the slope environment. Here we present a methodology to create outcrop models and integrate complementary types of data that provide new insights in sedimentology that were previously unattainable.

Abstract: The drilling of two core borings in the Neogene western Great Bahama Bank provided the basic descriptions of lithology for evaluations of the source and mode of deposition, diagenetic patterns, and the role of relative sea-level fluctuations. Comparison of these parameters with those of the Holocene revealed a consistent and general relationship between the position of sea level and the source and mode of deposition. The bulk of the slope sediment, more than 80%, was sourced by extensive offbank transport of fine-grained suspended sediment and minor pelagic sediment when the platform was submerged During sea-level lowstands, only minor amounts of reworked, coralgal sediment were deposited and rapidly cemented on the slope, whereas subaerial exposure surfaces developed on the platform and the margin. During the subsequent initial rise of sea level, marine hardgrounds developed on the slope and, in some cases, at the margin The marginal coralgal and skeletal shallow-water depositional environments were either reworked or retreated until sediment production was in equilibrium with sea level and offbank sediment transport of fines resumed. At least three different frequencies of sea-level fluctuations are recorded on the slope as sedimentary sequences, with a thickness ranging from meter scale up to approximately 170 m. At least five, and possibly seven, larger-scale sedimentary sequences ( A-G ), varying in age from late Miocene to Pleistocene, are recognized and correlated in both cores. Each sequence is characterized by distinct variations in composition, texture, and degree of cementation. At the margin, the depositional signature is less distinct and only the largest-scale sea-level fluctuations are recognized, local accumulation and removal by erosion of sediment at the shallow-water margin are suggested to obscure the recognition of sea-level-genera ted sedimentary packaging Two sequence boundaries are associated with global climatic events: (1) the expansion of the Antarctic ice Sheet and postulated fall of sea level of approximately 50 m at the Miocene-Pliocene boundary (sequence D ), and (2) the onset of the first post-Miocene cooling in the late Pliocene at approximately 3.2-2.8 Ma (sequence C ). The onset of first post-Miocene cooling (sequence C ) is associated with dramatic changes in depositional style: (1) from mixed skeletal and nonskeletal to predominantly peloidal, (2) increased rates of accumulation on the slope, or from predominantly aggrading to prograding, and (3) an increase in the frequency of sea-level fluctuations. it is suggested that these changes are associated with the change from a low-angle ramp to a flat-topped platform. Slope systems of well-documented ancient examples of carbonate platforms have usually steep flanks, are dominated by reef-margin-derived sand and rubble, and have a low potential of recording sea-level fluctuations. In contrast, as a result of intimate relationships between sediment sources, modes of accumulation, and relative position of sea level, the Neogene low-angle slope of western Great Bahama Bank contains a high-resolution archive of sea-level fluctuations.

Abstract: Independent analyses to date prograding carbonate platform-margin sediments have enabled development of an integrated chronostratigraphy of the west margin of the Great Bahama Bank (GBB). The chronostratigraphy permits interpretation of periods of deposition, nondeposition, and erosion on the platform and margin. The integrated chronostratigraphy is based on planktic foraminiferal biostratigraphy, calcareous nannofossil biostratigraphy, strontium-isotopestratigraphy, and magnetostratigraphy. Critical to interpreting each type of stratigraphy is a clear understanding of slope dynamics and the depositional system. The biostratigraphic data, in particular, are affected by the extreme dilution of key microfossils by platform-derived sediments during margin progradation. Because of this factor, the highest abundance of microfossils is restricted to thin units of pelagic sediment, deposited during temporary intervals when platform sediment supply was greatly reduced. However, these selective microfossil concentrations are also more likely to yield premature last-appearance datums and delayed first-appearance datums than the diluted intervals, because they represent short periods of time. Despite these problems, the microfossils provide critical age control. The detailed chronostratigraphy allows interpretation of a well-constrained platform-margin evolution. Core Unda, the more landward location, penetrated the oldest sediments (earliest late Miocene). The more seaward core Clino, although deeper, bottomed in younger sediments (latest Miocene). Three major progradational episodes were delineated using seismic stratigraphy, lithostratigraphy, and information on depositional age. Progradation occurred during the late Miocene, late early Pliocene, and latest Pliocene. In the Pliocene shelf and ramp setting margin progradation began during a highstand, but also occurred in a forced-regression-type situation during a fall in sea level. Rapid reef progradation occurred near the end of the Pliocene and early Pleistocene, when the platform had aggraded to a nearly horizontal surface. The transformation from a ramp-type platform topography to a flat-topped platform culminated in the middle Pleistocene. Age constraints across the west margin of GBB indicate that the seismic reflectors that constitute sequence boundaries are synchronous, within our age resolution. As expected in a slope setting, the sequence boundaries represent condensed time periods of both erosion and nondeposition. Downslope plankton-rich onlapping units are correlated to upslope marine hardgrounds and are thought to represent periods of falling sea level. Subsequent high-resolution dating of additional basinal and shallow-platform borings will provide a rich archive of paleoceanographic changes and will serve as a direct link between the deep-ocean and shallow-platform accumulations.

Abstract: During the Neogene, western Great Bahama Bank prograded more than 25 km into the Straits of Florida. This progradation occurred in pulses seen on seismic lines as distinct seismic sequences. Two core holes (Unda and Clino) were drilled through the proximal parts of these prograding carbonate sequences to test several fundamental questions in carbonate sedimentology, diagenesis, and sequence stratigraphy. The sequence strarigraphic objectives of the drilling were to (1) provide information on facies within the seismic sequences, (2) test if predicted facies changes coincide with seismic sequence boundaries and inferred systems tracts, (3) determine the ages of sequence boundaries, and (4) test whether the prograding sequences were sea-level controlled, and if so, provide information about the timing and amplitude of these relative changes in the later part of the Neogene. The cores at sites Unda and Clino penetrated eight seismic sequences that have a Mid/Late Miocene to Recent age. The sedimentary successions in the cores display a repetitive pattern of changing facies that can be related to changes in sea level. Sea-level falls are indicated on the slope by either hardgrounds, firmgrounds, or coarser-grained beds, whereas caliche horizons or karst surfaces document exposure on the platform. Aggrading shallow-water carbonates and thick fine-grained slope deposits are interpreted to be deposits of sea-level highstands. These lithologic indications of changing sea level coincide with the interpretation of the seismic sequence architecture. Seismic sequence boundaries correlate to horizons and/or sedimentary packages indicative of sea-level falls, whereas prograding pulses coincide with the thick slope sections deposited during sea-level highstands. In addition to the facies there is a strong correlation between diagenesis and the seismic sequences. This relationship can be seen in both the paragenetic sequences and the variations in mineralogy. In core Unda, all paragenetic boundaries coincide with sequence boundaries. In the slope section of Clino, mineralogical changes occur across diagenetic horizons, e.g., hardgrounds, which also coincide with seismic sequence boundaries. These surface-bounded changes in diagenesis give strong indications that changes in sea level alter the diagenetic potential of the carbonates within the sequence stratigraphic pattern. Petrophysical analyses document that the combination of changing facies and diagenesis across sequence and systems-tract boundaries produces the necessary impedance contrasts for seismic reflections along stratigraphic surfaces. The ages of the eight seismic sequence boundaries were determined using a combination of biostratigraphy, Sr-isolope stratigraphy, and magnetostratigraphy. The ages of the sequence boundaries are the same in both drill sites and do not cross seismic reflections, documenting that seismic reflections separate sedimentary packages of different ages. This age consistency confirms one of the basic assumptions of sequence stratigraphy, i.e., that seismic reflections are time lines and have chronostratigraphic significance. The duration for the deposition of the individual sequences is between 0.5 and approximately 1 Myr, indicating that the sequences are produced by third-order sea-level fluctuations. The sequence architecture reflects a long-term pattern of sea-level changes that is in good agreement with the known global sea-level history. A general lowering of sea level at the end of the Middle Miocene resulted in progradation of the platform and the development of a margin at drill site Unda. At about 8.9 Ma progradation was interrupted and the margin stepped back because of a sea-level rise. The lowering of sea level in the latest Miocene and the subsequent sea-level lowstand that lasted during most of the Messinian is recorded in Unda by the development of a reef on the former slope deposits and recognized on the seismic line as a major erosional unconformity at the margin. Backstepping of the platform margin in the early Pliocene indicates a rapid flooding, which is also a major transgression on the global chart. The high-amplitude sea-level changes in the Pleistocene are recorded as laterally stacked sequences with increasingly higher angles in the foresets.