Abstract Carbonate rock typing provides a vehicle to propagate petrophysical properties through association with geological attributes and, therefore, is critical for distributing reservoir properties, such as permeability and water saturation, in the reservoir model. The conventional approaches to rock typing have significant gaps in incorporating diagenetic processes, transferring rock types from core to log domain, accounting for fractures and using appropriate methodology to realistically distribute rock types in the static reservoir model. The workflow proposed in this paper addresses these issues in a comprehensive way by determination of petrophysical rock types (PRTs), which control static properties and dynamic behaviour of the reservoir, while optimally linking to geological attributes (depositional and diagenetic) and their spatial interrelationships and trends. This approach is novel for the fact that it: (1) integrates geological processes, petrophysics and Earth modelling aspects of rock typing; (2) integrates core and log scales; and (3) provides a flexible ‘road map’ from core to 3D model for variable data scenarios that can be updated with progressive changes in data quality and quantity during the life cycle of an asset. This paper introduces the rationale behind this workflow, and demonstrates its workings and agility through deployment in two large carbonate fields.

Abstract Carbonate submarine slopes have a tendency to be steeper than their siliciclastic counterparts. Commonly the stabilization potential by binding of slope sediment and early cementation of carbonates is invoked to explain this difference. However, differences and similarities between siliciclastic and carbonate slope systems with respect to their gross development, curvature, and angle of dip are only expressed if one evaluates slope settings that are affected by comparable extrinsic and intrinsic processes. Three basic types of slope profiles (planar, concave, and sigmoidal) are reviewed and their mathematical expressions (linear, exponential, and Gaussian, respectively) used to compare and contrast slope systems originating from various settings. Exponential slopes with sharp shelfbreaks develop if sedimentary base-level fluctuations are minor compared to slope progradation. Gaussian profiles develop as a result of rounding of the shelfbreak by significant base-level fluctuations, whereas linear profiles result from excess sedimentation creating an angle of repose system. Both carbonate and siliciclastic systems exhibit all three types of slope curvature and mutually comprise muddy and grainy as well as debris-dominated slopes. Similarities between continental slopes of siliciclastic passive shelf margins flooded during the Holocene transgression and nonrimmed, cool-water carbonate platforms are evident where deep shelves, low slope angles, and usually Gaussian slope profiles are typical. Lacustrine and proximal, active marine deltas compare with tropical carbonate platforms. Both have steep, exponential, and linear slope profiles and coarse sediments originating from shallow water depths. Exponential profiles are common on rimmed platforms because reefs are resistant to erosion and the platform edge is therefore relatively stationary vertically, thus forming a distinct platform-slope break. This also accounts for ice-covered margins because the grounding level of the ice limits vertical fluctuations. A special case for carbonates is the in situ accretionary slope factory dominated by microbial boundstone-dominated deep oligophotic “reefs” and linear slopes of rubble, boulders, and sand. However, in situ slope accretion and stabilization by itself does not necessarily explain the first-order linear profile. Because the slope factory is insensitive to light accretion by slope shedding occurs during both lowstands and highstands. In other words, when shallow-water carbonate production ceases, in situ carbonate production continues in the slope region, and the combined effort of sediment production and the resultant surplus allows the system to build up to the angle of shear and constantly prograde. Since the dominant sediment texture delivered by the slope factory is coarse rubble and boulders that yield high angles of repose, often the flanks are steep. A direct comparison are coarse-grained deltas, especially those that develop in fjords and Alpine lakes, where because of its proximity to the sediment source the inherent fast prograding system, which is dominated by a mixture of coarse sand and rubble, obtains steep, linear slopes. Clearly, while sediment properties may vary greatly, stark similarities in gross development, curvature, and angle are observed in comparable settings. As a consequence, morphometric attributes captured from seismic data have to be put in the context of the entire depositional system and basin setting to fully comprehend and predict sediment properties and depositional processes.

Abstract Outcrops of intact and seismic-scale Pennsylvanian (Serpukhovian to Moscovian) carbonate platforms in Asturias (NW Spain) were studied as analogues of the prolific subsurface reservoirs in the Pricaspian Basin (e.g., Tengiz Field). The Asturian platforms, which have been rotated 90° along the dip axis during Late Carboniferous thrusting, are visible on aerial photographs as kilometer–scale cross sections and have dimensions similar to their Pricaspian subsurface equivalents: a thickness between 1.5 and 2.0 km, a slope relief up to 850 m, slope angles up to 32°, and oblique–exponential clinoforms. A comparative study of stratal patterns, lithofacies, and petrophysical properties using aerial photography was initiated to: (1) develop a depositional model, (2) construct a seismic model for comparison with the Pricaspian subsurface, and (3) address the controls on slope declivity. Five general lithofacies–stratal pattern zones were observed: inner and outer platform, upper slope, lower slope, and toe–of–slope to basin. The platform zone has shoaling–upwards cycles with a transgressive interval of coated grainstone with oncoids, followed by normal marine algal boundstone and bioclastic grainstone to packstone and, near the top, restricted lagoonal peloidal packstone to grainstone with calcispheres. These cycles have a thickness between 2.5 and 15 m and can be traced from the platform break into the platform interior for at least 6 km. Crinoid–bivalve grainstone to rudstone intervals and lenticular mud mounds are present in the outer platform, a one– kilometer–wide zone near the platform break. Two distinctly different automicrite margins are recognized in the field: (1) low–angle slopes and ramps, deposited during the nucleation phase of the platform, of nearly pure micritic limestone, and (2) steep (26 to 32°) slopes where automicrite boundstone dominates the uppermost 300 m. Clotted peloidal micrite—automicrite—with sponges and fenestellid bryozoans, and crinoid rudstone intervals, dominate this zone. Below 300 m paleo–water depth, clast–supported lithoclastic breccia dominates the slope. Finally, below 600 to 700 m, argillaceous lime mudstone beds interfinger with grainstone to wackestone intervals of mostly platform– top–derived grains and thick intervals of upper–slope–derived breccia. Five major phases of platform development are recognized: (1) renewed flooding of the preexisting regional Serpukhovian platform, rucleation of a low-angle ramp with microbial mud deposits, aggradation and subsequent formation of a steep microbial cement boundstone margin, followed by nearly horizontal progradation (Bashkirian), (2) continued progradation with several aggradational phases (Bashkirian), (3) development of an extensive flat-topped shallow-water platform near the Bashkirian-Moscovian boundary followed by combined aggradation and progradation, (4) predominantly progradation, followed by (5) aggradation. On the steep upper slopes, over 30°, automicrite formation alternated with deposition of sand and rubble. Automicrite layers slid off and formed breccia tongues at the toe of slope whenever the shear strength of the substrate of loose sediment was exceeded. The steep slope angles were maintained by alternating automicrite growth stages and gravity-driven deposition and consequently inhibited the growth of large mud mounds. Calibration of lithofacies and stratal patterns in a large-scale platform outcrop with their potential seismic expression through synthetic seismic modeling shows great similarity with seismic data acquired in the Pricaspian subsurface. The integration and quantification of size-similar outcrop data is a first step in developing a powerful predictive tool for the exploration of the subsurface of the Pricaspian Basin.