Shoal Bodies Research Articles

Geologic characterization of the Lower Cretaceous upper Travis Peak and Pettet Formations in the Wright Mountain field, Smith County, East Texas basin: Potential reservoir analog for other Travis Peak and Pettet strata

The Wright Mountain field in Smith County, Texas, is a tight carbonate reservoir that produces from the Lower Cretaceous Pettet Formation. The Pettet Formation is dominantly made up of cyclic shallow-water shelfal carbonates associated with skeletal oolitic shoal complexes. These shoals are widespread hydrocarbon-reservoir targets, have poor to fair matrix reservoir quality, and are commonly sealed by transgressive argillaceous lime mudstones and wackestones. Complex regional paleotopography has significant impact on the regional distribution of these shoal bodies and overall depositional patterns. This investigation, therefore, aims to characterize the Pettet Formation and underlying fluviodeltaic Travis Peak Formation in the Wright Mountain field, located on the western flank of the Strickland High near the northeastern margin of the East Texas Basin, to assess reservoir parameters and to compare depositional patterns to existing shoal-complex studies centered on the top of the paleohigh in Rusk County. The Pettet section in Smith County consists of three fourth-order sequences (Pettet A–C), each of which shoals upwards into a shallow-water, high-energy environment near or within shoal complexes. The best reservoir lithofacies are the mixed skeletal-ooid and skeletal grainstones, as observed elsewhere in the Pettet Formation in Rusk County. However, texture and biota vary in the Wright Mountain field area as compared to the Rusk County area as there is abundant evidence of reworking and resedimentation in the Wright Mountain field area, suggesting a different depositional setting perhaps related to off-shoal deposition or greater depositional dip. Therefore, this field characterization study is important for constructing a regional depositional model and can be applied as an analog for equivalent systems with similar lithofacies and pore networks elsewhere in the northern Gulf of Mexico.

Carbonate Platform Reef-Shoal Reservoir Architecture Study and Characteristic Evaluation: A Case of S Field in Turkmenistan

Carbonate sediments are susceptible to many factors, such as paleostructure, diagenesis, and strong microbial alteration; as such, their sedimentary architecture still calls for further research. In this study, the reef and shoal bodies in the XVm and XVp layers of the Middle–Upper Jurassic Karlov-Oxfordian in the S gas field were used as the object, and the architecture of the reef-shoal facies was studied. Based on the idea of “vertical grading and horizontal boundary”, the interface characteristics of the 6th to 4th levels of reef-shoal bodies in the study area were summarized, as were four ways to determine the boundaries of reef-shoal bodies. Based on the dense well network, we quantitatively described the scale of each small layer of single reef shoal body through the point-line-surface method and established a geological database of the reef shoal bodies in the study area. In addition, we established the width and thickness of the reef shoal body and the empirical formula for relationships. The study of morphological characteristics of reef-shoal bodies and the degree of overlap revealed six architecture models of reef-shoal bodies. The vertical and lateral superimposed reefs were obtained by evaluating the reservoir space, pore throat characteristics, and physical property characteristics corresponding to various architecture models. The vertical and lateral superimposed shoals corresponded to large reservoir thicknesses. The petrophysical properties were good, and we concluded that the reef-shoal superimposed area was a sweet spot for reservoir development. We applied the research results of reef-shoal architecture reservoir characteristics to gas field development, and therefore improved well pattern deployment in the reef-shoal superimposed area. By comparing the test results of newly deployed horizontal wells with adjacent vertical wells, we confirmed that the selection of horizontal wells was better for gas field development. This study on the architecture of reef-shoal facies could guide the study of carbonate rock architecture.

A new scale shoal-forming model and natural gas exploration implications in Feixianguan Formation, northwestern Sichuan Basin, China

As an important exploration target in the Sichuan Basin, the Lower Triassic Feixianguan Formation has been previously interpreted to be deposited in the trough-platform margin system according to the genetic model of classic lithofacies paleogeography. However, this model does not apply to the northwestern Sichuan Basin. Accordingly, this study proposed a new genetic model that highlighted the carbonate ramp based on the outcrop sections, drilling and logging data, and seismic sections. A large-scale regional regression was found to occur in Sichuan Basin at the end of the Late Permian, leading to the evolution of the original Kaijiang-Liangping Trough into a very shallow tidal flat/lagoon environment at the beginning of the Early Triassic. The sedimentary pattern during the Early Triassic was different from that during the Late Permian. A total of six long-term cycles (LSC1-LSC6) were identified within the Feixianguan Formation. The deposition of LSC1-LSC3 witnessed the filling of the trough to a large extent, and the depositional periods of LSC4 and LSC5 were the main shoal-forming periods. The Feixianguan Formation in the study area was speculated to be deposited in the continuous continental carbonate ramp. With the intensified uplifting of the Longmenshan island chain and the Kangdian ancient land on the northwestern margin of the Upper Yangtze Platform, a large number of terrigenous materials were input into the study area from west to east, leading to the continuous eastward migration of the carbonate facies belt during the deposition of the Fei 1 and Fei 2 Members. Meanwhile, large-area inner-ramp shoal bodies were formed during the deposition of LSC5. It became possible to find a giant gas reservoir belt in the Feixianguan Formation of the northwestern Sichuan Basin and the results of this study provide a new complement to the existing classic trough-platform margin shoal-forming model.

Formation and distribution characteristics of Proterozoic\u2013Lower Paleozoic marine giant oil and gas fields worldwide

There are rich oil and gas resources in marine carbonate strata worldwide. Although most of the oil and gas reserves discovered so far are mainly distributed in Mesozoic, Cenozoic, and upper Paleozoic strata, oil and gas exploration in the Proterozoic–Lower Paleozoic (PLP) strata—the oldest marine strata—has been very limited. To more clearly understand the oil and gas formation conditions and distributions in the PLP marine carbonate strata, we analyzed and characterized the petroleum geological conditions, oil and gas reservoir types, and their distributions in thirteen giant oil and gas fields worldwide. This study reveals the main factors controlling their formation and distribution. Our analyses show that the source rocks for these giant oil and gas fields are mainly shale with a great abundance of type I–II organic matter and a high thermal evolution extent. The reservoirs are mainly gas reservoirs, and the reservoir rocks are dominated by dolomite. The reservoir types are mainly karst and reef–shoal bodies with well-developed dissolved pores and cavities, intercrystalline pores, and fractures. These reservoirs are highly heterogeneous. The burial depth of the reservoirs is highly variable and somewhat negatively correlated to the porosity. The cap rocks are mainly thick evaporites and shales, with the thickness of the cap rocks positively correlated to the oil and gas reserves. The development of high-quality evaporite cap rock is highly favorable for oil and gas preservation. We identified four hydrocarbon generation models, and that the major source rocks have undergone a long period of burial and thermal evolution and are characterized by early and long periods of hydrocarbon generation. These giant oil and gas fields have diverse types of reservoirs and are mainly distributed in paleo-uplifts, slope zones, and platform margin reef-shoal bodies. The main factors that control their formation and distribution were identified, enabling the prediction of new favorable areas for oil and gas exploration.

Characteristics of dolomite karstic reservoir in the Sinian Dengying Formation, Sichuan Basin

The Sinian dolomite reservoir in Dengying Formation was developed extensively in the Sichuan Basin, and typically was the facies-controlled dolomite karstic reservoir. The development of this kind of the reservoir was related with the special deposits, diagenetic environment as well as the multi-phase and multi-type dissolution and fracturing actions. The arid-hot/dry paleo-climate was favourable to develop the large-scale microbial reef-shoal bodies and mound–shoal bodies, which was characterized by the organic framework and the sea floor hardening, thus forming primary matrix pore-type reservoir. In the background of the arid-hot/dry paleo-climate, the water-rock interaction was weak, the mineral stabilization process was slowly, and the vadose pisoliths, aragonite and high-magnesium calcite were well preserved, therefore, this diagenetic environment especially was beneficial for pore preservation. Moreover, the syngenetic and penecontemporaneous dissolution, especially three phases of weathered crust karstification due to three episodes of the Tongwan movement, led to further dissolution of spaces and form numerous non-fabric selective dissolution fractures and dissolution caves along faults and fractures. Multi-phase fractures, especially the late Yanshanian-Himalayan tectonic fractures, resulted in connection among pores, vugs and dissolution caves, and improved quality of the reservoir tremendously.

Differences between the platform-margin shoal reservoirs and the platform-interior shoal reservoirs of the Middle Triassic Leikoupo Formation, Sichuan Basin, China

The carbonate shoal grainstone reservoirs of the Middle Triassic Leikoupo Formation in the Sichuan Basin provide a typical example of gas exploration in shoal carbonate reservoirs. The carbonate shoal reservoirs in Leikoupo Formation are of two types: platform-margin shoals and platform-interior shoals. Based on a comprehensive data analysis, and through analysis and correlation of grain shoal reservoir characteristics, it has been shown that there are significant differences between platform-margin and platform-interior shoal reservoirs in terms of the types of reservoir space, the relationship between porosity and permeability, and the pore throat structures. The pores in the platform-margin shoal reservoirs are mainly relict intergranular pores and intergranular dissolution pores, and their porosity and permeability show a good positive correlation. The throats are thick and the pore throats have a good distribution. Isolated intragranular solution pores and moldic pores are mainly developed in the platform-interior shoal reservoirs. There is a weak positive correlation between porosity and permeability, pore throats are less developed, and the pore throats have poor configurations. Differences in sedimentary environment and diagenetic evolution are the primary causes of the different reservoir characteristics of the two shoal facies. The water body in the platform-margin shoal deposits had a higher energy than that in the platform-interior shoal deposits, and its wave base was deeper that of the platform-interior shoals. The shoal bodies at the platform margin are much thicker than those in the platform interior. The grainstones of platform-margin shoals are much better sorted. The rocks have undergone compaction during most post-depositional stages. The thicker grainstones of the platform-marginal shoals preserved their original intergranular pores better than the lower-thickness grainstones in the platform-interior shoals. This was favorable for formation of intergranular dissolution pores during later burial. In contrast, the grainstones of the platform-interior shoals are less thick and early intergranular cements are well developed, resulting in large-scale loss of the original intergranular pores. However, as a result of selective dissolution by early fresh water in the meteoric diagenetic environment, intragranular solution pores and moldic pores were readily formed. Thus, the platform-margin shoal reservoirs show preservation of the original intergranular pores enhanced by burial dissolution, but the platform-interior shoal reservoirs are mainly the result of early selective dissolution by meteoric fresh water.

Numerical simulation of fluid-flow processes in a 3D high-resolution carbonate reservoir analogue

A high-resolution three-dimensional (3D) outcrop model of a Jurassic carbonate ramp was used in order to perform a series of detailed and systematic flow simulations. The aim of this study was to test the impact of small- and large-scale geological features on reservoir performance and oil recovery. The digital outcrop model contains a wide range of sedimentological, diagenetic and structural features, including discontinuity surfaces, shoal bodies, mud mounds, oyster bioherms and fractures. Flow simulations are performed for numerical well testing and secondary oil recovery. Numerical well testing enables synthetic but systematic pressure responses to be generated for different geological features observed in the outcrops. This allows us to assess and rank the relative impact of specific geological features on reservoir performance. The outcome documents that, owing to the realistic representation of matrix heterogeneity, most diagenetic and structural features cannot be linked to a unique pressure signature. Instead, reservoir performance is controlled by subseismic faults and oyster bioherms acting as thief zones. Numerical simulations of secondary recovery processes reveal strong channelling of fluid flow into high-permeability layers as the primary control for oil recovery. However, appropriate reservoir-engineering solutions, such as optimizing well placement and injection fluid, can reduce channelling and increase oil recovery.

Middle to Upper Khuff (Sequences KS1 to KS4) outcrop-equivalents in the Oman Mountains:Grainstone architecture on a subregional scale

ABSTRACT The upper part of the Saiq and lower part of the Mahil formations in the Oman Mountains represent outcrop time-equivalents to the highly prolific, hydrocarbon-bearing subsurface Middle and Upper members of the Khuff Formation (K4-K1 reservoir intervals). In this study, four outcrops sections on the northern flank of the Oman Mountains (Al Jabal al-Akhdar region) are sedimentologically documented and integrated with the sequence-stratigraphic scheme initially developed at the Saiq Plateau reference section. The focus of this study is the description of the distribution and textural variation of grainstones as potential reservoir facies on a subregional (ca. 60 x 40 km) scale. Stratigraphic cross-sections are constructed based on two sequence-stratigraphic orders: (1) one second-order supersequence (DS2 18) that provides a well-constrained general framework and (2) four third-order depositional sequences (KS1-KS4), within which subtle temporal and spatial variations of grainstones occur. From these correlations predictive rules and correlation lengths of shoal bodies are extracted. The presence of Khuff grainstones is strongly governed by stratigraphic position. Thicker and more abundant grainstones are present during the early transgressive (KS4) and late regressive (KS1) portions of the supersequence. Thinner and less abundant grainstones are present during the late transgression (KS3 and lower KS2) and early regression (upper KS2). They are absent around the second-order zone of maximum flooding (middle KS2). High lateral continuity of correlated cycle sets is observed, suggesting the absence of significant tectonic activity of the area during the Late Permian and Early Triassic. Integrated litho-, bio-and sequence stratigraphy provides a robust framework for correlation on a regional scale (ca. 700 km). The Oman Mountains area shows a more distal facies pattern on the Khuff platform compared with other Khuff reservoir sections in the region. This is especially evident around KS2 maximum flooding with muddy foreshoal and offshoal deposits in contrast to mainly oolitic shoal deposits in the Musandam (UAE) and offshore Fars (Iran) area.

A new workflow for carbonate reservoir modelling based on MPS: shoal bodies in outcrop analogues (Triassic, SW Germany)

Abstract This study presents a workflow for 3D modelling of carbonate reservoirs using multiple-point statistics (MPS) in the framework of a pre-existing model. It consists of the following steps: (1) applying a hierarchical classification scheme for carbonate geobodies; (2) based on this classification, retrieval of relevant data from Carbdb, a novel software to manage a database of analogue studies; (3) construction of training images based on the retrieved data from Carbdb; and (4) using the training images when building a 3D reservoir model with MPS. MPS makes use of training images to capture depositional patterns, which will then be reproduced during the stochastic simulations. Carbdb provides a library of quantitative data such as dimensions, geometries and the distribution pattern of geobody analogues necessary for building training images. The MPS workflow was applied to carbonate shoal bodies from a reservoir analogue, the Muschelkalk in SW Germany. Present-day shoal bodies from the Arabian Gulf were retrieved from Carbdb as possible modern analogues to generate training images. The realizations of this MPS approach are compared with a previously established 3D geocellular model that was built deterministically by interactive facies modelling. The MPS simulations produced geologically more realistic facies distributions with higher facies heterogeneity, similar to the depositional patterns observed in modern analogues.

Three-dimensional facies modeling of carbonate sand bodies: Outcrop analog study in an epicontinental basin (Triassic, southwest Germany)

This article is the first part of an integrated study to characterize the anatomy and geometries of carbonate shoal reservoir bodies in epicontinental settings. It is based on outcrop analog data from Triassic layer-cake carbonates in the south German Basin, which were deposited along an epicontinental, very gently inclined carbonate ramp. The database of this study consists of 56 measured sections supplemented by six shallow cores and wireline logs, covering an area of 25 by 36 km (15 by 22 mi). The potential reservoir bodies consist generally of midramp shoal and shoal-fringe facies types, which are composed of skeletal and oolitic carbonate packstones and grainstones with significant amounts of porosity. The upper Muschelkalk is composed of a lower transgressive and an upper regressive interval; within these, shoal bodies show not only similarities but also major differences in character, geometry, and distribution. These reflect the conditions of an epeiric system, which reacts strongly to small changes in accommodation. The accommodation seems to be mainly triggered by the interaction of hierarchically organized large-, medium-, and small-scale relative sea level changes and a subtle paleorelief. At the level of medium-scale cycles, distinct two-dimensional cyclicity styles can be differentiated, which record the lateral facies distribution along the depositional gradient. Different styles of medium-scale cycles include different types of shoal bodies: (1) transgressive crinoidal shoal style: shoal bodies (mean width: 21 km [13 mi], mean length: 37 km [23 mi], mean thickness: 4.2 m [13.7 ft]) with low facies differentiation, deposited on top of subtle paleohighs; (2) skeletal sheets style: thin reservoir sheets (mean width: 5.1 km [3.1 mi], mean length: 11.1 km [6.8 mi], mean thickness: 0.3 m [0.9 ft]) of reservoir facies on top of the paleohighs; (3) regressive oolitic shoal style: shoal bodies (mean width: 14.6 km [9.0 mi], mean length: 20.8 km [12.9 mi], mean thickness: 0.69 m [2.2 ft]) with high facies differentiation on the flanks of paleohighs; and (4) low-accommodation style: patchy and mosaic distribution of shoal bodies (mean width: 11.2 km [6.9 mi], mean length: 26 km [16 mi], mean thickness: 0.69 m [2.2 ft]). All data were loaded into three-dimensional (3-D) modeling software to distribute 14 facies types and model the 3-D stratigraphic architecture. The resulting facies distribution implies that volume and dimensions of the shoal bodies are mainly controlled by the combination of stratigraphic cycles and a subtle paleorelief, which is indicated by overall thickness changes of succession. High-energy shoal facies types occur only in and around areas with a reduced overall thickness, whereas areas with a thicker development are dominated by low-energy, muddy facies types. All observations combined point to the presence of a subtle paleorelief, which could be induced by slight differential subsidence of inherited paleotectonic basement blocks. On the kilometer to regional scale, previous studies suggest simple layer-cake stratal patterns. However, in the full 3-D view, the apparent layer-cake stratigraphy turned out to be a pseudolayer cake, with very gentle (0.01–0.001 dip) clinoform geometries. These very subtle offlapping, pinch-out geometries may have been overlooked in epeiric reservoir systems elsewhere.

Controls on morphology and sedimentology of carbonate tidal deltas, Abacos, Bahamas

Although the physical processes controlling siliciclastic shoal bodies are reasonably well-understood, the sedimentary processes that shape carbonate tidal shoal systems are poorly constrained. The tidal deltas of the northwestern Abaco Islands, Bahamas, provide a unique opportunity to explore the relationships between physical and sedimentary processes in a variety of sizes and shapes of tidal deltas. The length of tidal deltas range in size from ~ 350 m to 2 km (from the inlet to the termination of the delta lobe) and shoal crests consist primarily of well-sorted ooid sands (mean grain size between 400 and 500 μm; McManus sorting factor ≤ 0.70). Coated grains are rare off the shoals, and sorting decreases with distance from the shoal crests. Velocities in the rocky inlets can exceed 2 m/s in the daily tidal cycles, and the systematic distribution of grain types reflects tidal velocity variations. Areas with the highest velocities (≥ 0.80 m/s) correspond to hard, rocky bottoms with some skeletal grains. These regions of elevated velocities are focused within the inlets, where jets of high velocity currents form during each tidal cycle. These current jets extend outward from the inlet throat, and velocities decrease outwards as the flow disperses. Velocities between 40 and 80 cm/s occur in the regions with abundant ooids, and peloids are most abundant in the lowest velocity regions (< 0.40 m/s) in the tidal delta system. At a larger scale, inlet tidal prism and delta size are closely correlated in these tidal deltas, as in their siliciclastic counterparts. The results of this study illustrate a clear relationship between quotidian physical processes and their geomorphic and sedimentologic products in a carbonate system. In this context, carbonates appear to share many geomorphic similarities with siliciclastic analogs. The results represent an important step beyond qualitative facies interpretations and towards quantitative, specified environmental and sedimentologic characterizations for carbonate systems.

FACIES AND POROPERM CHARACTERISTICS OF A CARBONATE SHOAL (MUSCHELKALK, SOUTH GERMAN BASIN): A RESERVOIR ANALOGUE INVESTIGATION

Outcrop analogue investigations contribute to the understanding of the architecture and behaviour of subsurface hydrocarbon reservoirs. This study was designed to provide input for static reservoir modelling of shoal bodies on carbonate ramps.We investigated an oolitic and shelly carbonate shoal of Late Muschelkalk (late Ladinian) age, about 30 km long and 15 km wide, exposed in a series of quarries in southern Germany. The dimensions of this shoal body correspond to both modern analogues in the Persian Gulf and to oilfields in the Middle East.Fifteen outcrop locations were chosen to cover the entire shoal. A regional correlation based on facies, sequence analysis and outcrop gamma‐ray logs was used as a framework for mapping the distribution of shoal (i. e. reservoir) facies and associated reservoir properties.Three major depositional environments were recognized: foreshoal, shoal and backshoal. In each environment, specific vertical facies successions composed of thick shallowing‐upward units and thin deepening‐upward units forming metre‐scale cycles were deposited. These small‐scale cycles are stacked and form large‐scale shallowing and deepening trends. Shoal facies developed preferentially in the regressive parts of small‐scale cycles and were found to be laterally continuous over distances of at least several kilometres.Porous intervals within the shoal facies are concentrated along the tops of the small‐scale shallowing‐upward cycles. The porous intervals in successive small‐scale cycles increase in thickness and in absolute values of porosity upwards through the large‐scale shallowing trend. Separate vug pore types are characterized by relatively low permeabilities (0 to about 10 mD) in spite of the high porosity (up to 20%). The combined occurrence of separate vug and interparticle porosity leads to higher permeabilities (tens of mD).Isolines of porosity, permeability and net/gross follow the contours of the carbonate shoal complex. The highest values of porosity appear more or less at the centre of the shoal, while the highest values of permeability are found at the shoal margins. The distribution of maximum porosity and maximum permeability zones are thus controlled by the cyclic seaward‐stepping and landward‐stepping of the shoal body.The systematic variations in extent, stacking pattern and related petrophysical characteristics of the carbonate shoal bodies may be used for modelling shoal reservoirs in producing oilfields.

Abstract: The predictive relationship of productive San Andres carbonate shoal bodies as determined from outcrop and subsurface study

Abstract: The predictive relationship of productive San Andres carbonate shoal bodies as determined from outcrop and subsurface study