Outer Ramp Carbonate Research Articles

Microfacies analysis and Depositional Basin Model of the Upper Cretaceous, Kurdistan Region, Northern Iraq

This study focuses on the Upper Cretaceous system in the Zagros fold and thrust belt in the Kurdistan Region / Iraq. The impression of this study is created by the variety of formations, facies change, and age of the Cretaceous formations, as well as the different consequences of the contact situation between the formations, particularly the Qamchuqa-Bekhme and Kometan-Shiranish contacts formations. Through investigation, evaluation, and combination of results from Lithostratigraphy and Microfacies of seven sections in various locations of KRI/ Iraq, new paleogeography of the Late Cretaceous of Northern Iraq is constrained. The microfacies of entire Late Cretaceous formations in different sections outcome 16 sub-microfacies types demonstrating open marine deep shelf environment of Shiranish Formation, the entire reef (back-reef, fore-reef, reef body) environment of Bekhme Formation and pelagic open marine ramp depositional environment of Kometan Formation. The output results of stratigraphy and microfacies of Late Cretaceous depositional successions in the Kurdistan Region are paleogeography maps reconstruction. During the Turonian to Early Santonian periods, the Kurdistan Region and northern Iraq were pre-subduction basins due to the presence of open marine outer ramp carbonate of the Kometan Formation in the east (Sulaimaniyah province exceeding Hizop area) and inner shelf of Tanuma and Khasib formations in the center and west (Erbil province). Major regression and unconformities covered the West of the study region during the Middle-Late Santonian; the result of this regression degraded the entire inner shelf facies that deposited during Turonian-Early Santonian periods. During the Early-Middle Campanian, a foreland basin emerged in the extreme northeastern and the Kurdistan Region, which was covered by open marine of the Shiranish Formation and reef platform of the Bekhme Formation. The Kurdistan Region was an active foreland basin margin in the Upper Campanian-Maastrichtian, with a greatly expanded Tanjero Formation, an Aqra reef in the north, and an open marine Shiranish Formation in the center and west.

The origin, differential diagenesis and microporosity characteristics of carbonate mud across a late Paleogene ramp (Iraqi Kurdistan region)

Microporosity is common in many Middle Eastern carbonate reservoirs and various interpretations have been offered to explain its development, especially in Cretaceous reservoirs. In this study outcrops of late Paleogene carbonates of the north-eastern Iraq have been studied to assess the variation of porosity across a carbonate ramp and the factors that control its distribution. The depositional system of this late Paleogene succession was a homoclinal ramp dipping SW. Inner-mid ramp carbonate facies are massive to thick bedded units, mostly composed of packstone to grainstone, characterized by high diversity of benthic foraminifers, red algae, corals, gastropods and bivalves. Outer ramp carbonate facies are thin bedded wackestone to calcimudstone. In terms of matrix-dominated facies two distinct rock fabrics and pore systems have been identified: low porosity inner-mid ramp facies and higher porosity outer ramp facies. The inner-mid ramp carbonates show evidence of neomorphism and recrystallization of matrix mud which forms larger crystal size (average 3–4 μm) with smaller pore size (0.5 μm or less) and average porosity (8.7%). The outer ramp mud matrices exhibit the major component of planktic foraminifera and coccolith debris have smaller crystal size (average 1 μm) with larger pore size (1–2 μm), the average porosity (13.1%). For the inner-mid ramp matrices carbon and oxygen (C & O) stable isotopic values are negative indicating likely meteoric diagenesis. The relatively low strontium (Sr) and magnesium (Mg) values and high manganese (Mn) implies likely recrystallization and replacement of the marine precursor, probably high Mg-calcite and aragonite. In the outer ramp facies, the presence of coccolith and planktic foraminiferal debris indicates less diagenetic modification; the C & O stable isotopic values show less altered range with very low Mn and relatively high Sr and Mg also supporting less alteration. The textural and geochemical evidence supports an origin from a more stable, likely low Mg-calcite precursor.

High resolution sequence stratigraphy of the Middle–Late Triassic Al Aziziyah formation, northwest Libya

This study offers the first integration of facies distribution, isotope and sequence stratigraphy studies of the Middle-Late Triassic (Ladinian-Carnian) Al Aziziyah Formation at its exposure at the Jifarah Basin of northwest Libya. Eight sections were measured along a depositional dip cross-section of the Al Aziziyah Formation. The Al Aziziyah Formation is a 2nd-order sequence, consisting of mid-ramp and outer ramp carbonate facies, with inner ramp facies restricted to the southernmost sections. The inner ramp peritidal facies indicate arid depositional system characterized by mudcracks, evaporite nodules and abundant cryptalgalaminites. Ramp crest shoals were predominantly peloidal and oolitic packstone-grainstone. However, mid-ramp facies show low to high-density bioturbation. Outer ramp is thin-medium hummocky cross-stratified beds, and interbedded shale. Changes from fine sandstone and red clay to carbonate mark the transition from the underlying Kurrush Formation to the Al Aziziyah Formation. In the most updip location, the transition from the Al Aziziyah Formation to the overlying Abu Shaybah Formation is an iron-rich, phosphatic surface. Seven sequences (S1-S7) within the Ghryan Dome section were identified based on facies stacking patterns, field observations and carbon stable isotope profiles. North of the Ghryan Dome section, there are three mainly subtidal sequences (S8S10) that may not correlate to the south. Higher-order (4th-5th) parasequences are present in the Al Aziziyah Formation, but correlating these parasequences between sections is difficult due to internal erosional discontinuities or autocyclicity.

Architecture of a Neoproterozoic intracratonic carbonate ramp succession: Wynniatt Formation, Amundsen Basin, Arctic Canada

The Neoproterozoic Wynniatt Formation, part of the upper Shaler Supergroup, is exposed in the Minto Inlier of Victoria Island, Canada, and was deposited in the intracratonic Amundsen Basin. The unit consists of a southwest-thickening (480 to 1000m over ~300km) shallowing-upward succession of three carbonate ramp sequences separated by regional unconformities. In ascending order: 1) inner to outer ramp carbonate facies, gradationally overlain by siliciclastic rocks of a pro-delta slope setting; 2) inner to mid-ramp subtidal carbonate facies, including a regional stromatolitic barrier system; and 3) outer ramp carbonate (gravity flow) facies overlain by shallowing-upward subtidal to intertidal, mixed siliciclastic-carbonate inner ramp facies. Spatial arrangements of nineteen lithofacies illustrates that each carbonate ramp sequence represents part of a distally steepened, storm-dominated carbonate ramp, with an interval of deep-water carbonate rocks coincident with oceanic restriction that elevated salinity. Migration of depocentre loci for successive ramp stages reflects changing patterns of subsidence. This may identify far-field extensional effects in this intracratonic basin because ages of the lower (~850Ma) and middle (~761Ma) formation bracket initiation of supercontinent (Rodinia) break-up. Our work offers an improved sedimentary framework for interbasinal correlation with coeval Neoproterozoic basins. It highlights temporal changes in carbonate facies compared to older carbonate successions in the Shaler Supergroup, and it defines depositional context for the Tawuia-Chuaria assemblage zone, providing important interbasinal biostratigraphic correlation.

Architectural Characterization and Three-Dimensional Modeling of a Carbonate Channel Levee Complex: Permian San Andres Formation, Last Chance Canyon, New Mexico, U.S.A.

Abstract Complete understanding of outcrop-scale stratal architectures requires extrapolation of two-dimensional depositional cross sections into three dimensions. This study integrates a high-resolution digital outcrop model with outcrop observations to create a three-dimensional geologic model of distal outer-ramp carbonate stratigraphy. Data were taken from sinuous canyon-wall exposures of the Permian San Andres Formation of Last Chance Canyon, New Mexico. A series of laterally extensive carbonate benches previously interpreted as constructional sponge mud mounds are here modeled as a channel–levee complex that is characterized by an alternating history of aggradation punctuated by erosional sediment bypass. Levee mudstones have an erosional base, contain dolomitized peloidal thin beds, and have limited faunal constituents. Primary levees were initiated as a carbonate apron and transformed into a low-relief channel–levee system as a result of deposition by dilute turbidity currents. Subsequent sediment bypass incised through the pre-established channels and into the underlying facies. The fill consists of skeletal packstone lags and fine peloidal packstones, which are correlated to coral- and sponge-bearing secondary levee deposits that drape the primary levees. Debrites, also found within some channelized areas, are thought to represent channel-margin oversteepening and slumping. Channel widths range from approximately 300 to 800 m, and channel-to-levee relief reaches up to 40 m. Three-dimensional surface models representing architectural elements of the channel–levee complex confirm outcrop observations. Surfaces that were modeled as a best fit to the three-dimensional outcrop tracings reveal development of low-sinuosity channels in previously established bathymetric lows created by earlier phases of sediment bypass and erosion. Sequence stratigraphic interpretations suggest that the channel–levee complex developed during the highstand part of a long-term transgression. The overall transgressive setting led to generation of substantial quantities of carbonate mud in more proximal areas of the ramp. Accumulated mud in the source region was transported down depositional dip during highstand parts of high-frequency cycles and resulted in growth of the levees. Carbonate channel–levee complexes are not likely to form during highstand sequences in which the source sediment is grain-dominated. Leveed channels are a viable component of deep-water carbonate settings and may be more common than has been previously recognized. Complex stratal architectures found in this channel–levee complex could be analogous to apparently gullied slopes of muddy carbonate ramps, especially those developed in transgressive sequences.

Shallow to deep-water deposition in a Cratonic basin: an example from the Proterozoic Penganga Group, Pranhita–Godavari Valley, India

The unmetamorphosed Proterozoic succession dominated by deep-water lithographic limestone and shale in the western flank of the Pranhita–Godavari Valley is designated as the Penganga Group. The succession in different parts of the Valley includes the Pranhita Sandstone (25–400 m), the Chanda Limestone (300 m), and the Sat Nala Shale (>2000 m) in ascending order. The Pranhita Sandstone and the Chanda Limestone reveal considerable variations in the character of the stratal packages and depositional settings from Mancherial in the south to Adilabad in the north. The Sat Nala Shale in both southern and northern outcrop belts is completely devoid of sand. It is brown to purple in colour and resembles present-day deep-water mud deposits. In the Mancherial area, the Pranhita Sandstone consists of 25–400 m thick conglomerate, pebbly red arkose and quartzose sandstone succession of coastal alluvial fan to shallow shelf origin. The Chanda Limestone is micritic and locally includes interbedded lenses of cross-stratified quartzose sandstone in the lower part. The depositional milieu varies from shallow shelf to below wave base outer ramp carbonate platform. Around Adilabad, the Pranhita Sandstone (25 m) lacks the conglomerate–pebbly arkose association at the base and comprises only quartzose sandstone and shale of shoreface to muddy shelf settings. The Chanda Limestone is essentially micritic but, in contrast to the Mancherial area, includes several interbedded intervals of slope-related, autoclastic debris flow limestone conglomerates and calciturbidites, and represents deep, outer ramp to slope and basinal settings. A predominantly deep-water micritic limestone and deep-water shale succession suggests that the Penganga basin evolved to a vast, deep epicratonic sea connected to an open ocean. The absence of a coastal alluvial fan association at the lower part of the Pranhita Sandstone and presence of a slope to basinal association in the Chanda Limestone in the northern outcrop belts around Adilabad indicates a northward transition from coastal to deep shelf, slope and basinal conditions and a northerly paleoslope of the basin.

Modeling Reservoir Heterogeneity Within Outer Ramp Carbonate Facies Using an Outcrop Analog, San Andres Formation of the Permian Basin

Variably cyclic, fusulinid-rich, outer ramp facies of the Permian San Andres Formation are exposed along the Algerita escarpment, Guadalupe Mountains, New Mexico. We have used the outcrop exposures and cored wells drilled adjacent to the outcrop to assess reservoir- and interwell-scale variability of permeability as a potential analog for carbonate reservoirs in the Permian basin and elsewhere. Permeability distribution was evaluated using a field permeameter and conventional measurements on small core plugs taken along vertical and horizontal outcrop traverses and from the slim-hole cores. Permeability is related to the variable development of depositional cycles within a small-scale sequence and also to diagenetic overprinting. Geostatistical models of permeability variation, honoring the geologic and petrophysical data, were constructed and input into a waterflood simulator to understand the interactions between heterogeneity and flow. Variograms constructed from conventional plug analyses of permeability for a vertical outcrop transect contain a significant small-scale signal (noise), as seen in the high nugget effect (approximately 50% of the overall sample variance), and have correlation ranges of less than 4.5 m (15 ft). Different vertical variogram characteristics are displayed by cyclic and less distinctly cyclic parts of the San Andres. Variograms constructed for horizontal transect data from three distinct stratigraphic units have nearly identical properties. Overall, the ranges of correlation are short (3-3.5 m; 10-12 ft) when compared to typical interwell distances, supporting a nearly uncorrelated and highly variable permeability model. Using observed short ranges of vertical and horizontal correlation and honoring the vertical transect data, cross sectional, conditionally simulated permeability fields were generated and used in simulated waterfloods to investigate the sensitivities to an oil recovery model and overall fluid injection rate for this style of stratigraphy and cyclicity. Cyclic parts of the section are characterized by a potential for early water breakthrough and relatively high vertical sweep efficiencies. Within the less distinctly cyclic section, waterflood fronts have a fingerlike profile and vertical sweep efficiency is generally poorer.

Permeability and Fluid-Flow Patterns in Variably Cyclic Outer-Ramp Carbonates, San Andres Formation, Guadalupe Mountains, New Mexico, Part 2: Spatial Permeability Distribution and Fluid-Flow Simulation

Permeability and Fluid-Flow Patterns in Variably Cyclic Outer-Ramp Carbonates, San Andres Formation, Guadalupe Mountains, New Mexico, Part 2: Spatial Permeability Distribution and Fluid-Flow Simulation

Permeability and Fluid-Flow Patterns in Variably Cyclic Outer-Ramp Carbonates: San Andres Formation, Guadalupe Mountains, New Mexico, Part 1: Geologic Framework and Permeability Controls

Permeability and Fluid-Flow Patterns in Variably Cyclic Outer-Ramp Carbonates: San Andres Formation, Guadalupe Mountains, New Mexico, Part 1: Geologic Framework and Permeability Controls

Oolitic sandbody depositional models and geometries, Mississippian of southwest Britain: implications for petroleum exploration in carbonate ramp settings

A 1000 m thick early Mississippian carbonate supersequence, the “Carboniferous Limestone” of southwest Britain, consists of three third-order depositional sequences. These comprise parasequences in various configurations, and the whole forms a carbonate ramp stack. Within this framework five major oolitic carbonate sandbodies developed: (a) Castell Coch Limestone, (b) Stowe Oolite, (c) Brofiscin Oolite, (d) Gully Oolite, and (e) High Tor Limestone. The depositional regime was storm- and wave-dominated throughout and the major sandbodies represent a range of progradational carbonate beaches, barriers and detached subtidal shoals. Analysis of the three-dimensional shapes and distribution of these five examples shows that they evolved to produce three major carbonate sandbody geometries: (a) strings, (b) sheets, and (c) wedges. These geometries are characterised using the five field examples and offered as a template which may assist in the exploration and reservoir modelling of petroleum-rich high-energy ramp systems. Progradation, for up to 40 km, of barrier islands (Stowe Oolite) and beach-ridge plains (Gully Oolite Formation) generated strings, and “thick” sheets individually up to 10–20 m thick. “Thin” shoreface-retreat carbonate packstone/grainstone sheets up to 5 m thick (High Tor limestone) developed during transgressions as veneers across flooding surfaces. These are comparable with sheet sands developed in siliciclastic shelf depositional systems. Progradation, for up to 30 km, and vertical aggradation of shoreline-detached oolite shoals (Castell Coch limestone, Brofiscin Oolite), generated basinwards-expanding or thinning wedges up to 30 m thick. Tectonically controlled stacking of strandplain sheets produced a composite carbonate sandbody up to 80 m thick (Gully Oolite). The intrinsic (sedimentary) and extrinsic (eustacy, tectonism, climate) factors which controlled these sandbody geometries are addressed. Establishing the positions of the sandbodies accurately within depositional sequences allows them to be located within inferred seismic sequence geometries and provides one possible solution to the difficult problem of predicting carbonate facies distribution in subtle stratigraphic plays. In this ramp system, the most homogeneous sandbodies (up to 30 m grainstones), with greatest reservoir facies potential, are represented by shoal-belt wedges. Potential grainstone reservoir facies in the prograding shorelines are limited to the upper parts of individual shoreface sequences (max. 10 m grainstones). For shoreline carbonate sandbodies, the greatest reservoir and stratigraphic trapping potential exists in the earliest ramp parasequences where enveloping offshore sediments are siliciclastic mudstones. In later stages, potential seals are likely to be less reliable, low-porosity outer ramp carbonates.

Outer ramp cycles in the Upper Muschelkalk of the Catalan Basin, northeast Spain

The Upper Muschelkalk (Triassic) of the Catalan Basin, eastern Spain, was deposited upon a carbonate ramp of homoclinal type located in an intracratonic setting. The Rasquera Unit in the Upper Muschelkalk mostly consists of outer ramp carbonates arranged in upward shallowing cycles. Five cycles from 1.5 to 12 m thick are recognised. Each cycle shows an upward coarsening of grain-size, an upward increase in bed thickness, and an upward change in fossil content, reflecting an upward shallowing of the environment. In the complete cycle, marlstone and shale pass up into marlstone with thin-bedded line mudstones containing pelagic bivalves. The succeeding thin-bedded and nodular limestones with shaley partings contain a more varied fauna and are bioturbated; they pass up into thick-bedded skeletal wackestones. Packstone-coquinas locally cap the cycles and have a diverse benthic fauna, as well as the alga Tubiphytes occurring as bioclasts, encrustations around skeletal grains and forming microbuildups. Although some of the thin beds were deposited during weak storms, there is little evidence of intense storm activity. Petrological, sedimentological and palaeontological criteria, and the geological setting, allow the recognition of distal, intermediate and proximal facies on a relatively deepwater carbonate ramp.