The 10 most-cited articles in AAPG Bulletin, 2022

ABSTRACT In this paper, we present comprehensive data on stress orientation and relative magnitude in areas throughout North America where unconventional oil and gas are currently being developed. We find excellent agreement between maximum horizontal principal stress (SHmax) orientations over a wide range of depths, using multiple methods. In all basins studied, we observed coherent stress fields that in some cases vary systematically from one part of a basin to another. In the Appalachian Basin in the eastern United States, SHmax is oriented northeast–southwest to east-northeast–west-southwest and the style of faulting is compressive, transitioning from reverse faulting in eastern Pennsylvania and New York to principally strike-slip faulting in western Pennsylvania, Ohio, and West Virginia. In the midcontinent, central Oklahoma is characterized by an approximately east–west SHmax direction and strike-slip faulting. The Fort Worth Basin in northeastern Texas is characterized by normal–strike-slip faulting and a north-northeast–south-southwest SHmax direction. In the Midland…

ABSTRACT Quantifying the petrophysical and geomechanical properties of gas hydrate reservoirs is essential for understanding the natural hydrate system and predicting gas production behavior for future resource development. Pressure-core analysis tools were used to characterize methane hydrate–bearing sediments recovered from the Gulf of Mexico Green Canyon Block 955, under an international collaboration with The University of Texas and the National Institute of Advanced Industrial Science and Technology. Pressure-core samples were successfully transferred from Austin, Texas to Sapporo, Japan. Index property measurements (grain size, grain density, hydration number, gas composition, thermal conductivity), along with triaxial compression, consolidation, and permeability tests with a nuclear magnetic resonance (NMR) analyzer were conducted. Compression tests at different strain rates confirmed a strain rate dependence for hydrate-bearing sediment, and an equation for predicting strength as a function of hydrate saturation and strain rate is proposed. Compression and swelling indices…

Abstract The ca. 1.5–1.3 Ga Roper Group of the greater McArthur Basin is a component of one of the most extensive Precambrian hydrocarbon-bearing basins preserved in the geological record, recently assessed as of 429 million bbl oil (68 million cubic meters of oil) and between 8 and 118 TCF (222.56 billion cubic meters) of gas (in place). It was deposited in an intracratonic sea, referred to here as the McArthur-Yanliao gulf. The Velkerri Formation forms the major deep-water facies of the Roper Group. Trace metal redox proxies from this formation indicate that it was deposited in stratified waters, in which a shallow oxic layer overlay suboxic to anoxic waters. These deep waters became episodically euxinic during periods of high organic carbon export. The Velkerri Formation has organic carbon contents that reach ∼10 wt. %. Variations in organic carbon isotopes are consistent with organic carbon enrichment being associated with increases in primary productivity and export, rather than flooding surfaces or variations in mineralogy. Although deposition of the Velkerri Formation in an…

ABSTRACT Excess permeability in carbonate reservoirs is typically related to the presence of dual-porosity multiscale nonmatrix features that can result in early water production or breakthrough of injected fluids. Therefore, early recognition of nonmatrix deeply impacts field development strategies. We use publicly available data from the pre-Salt of Brazil to screen for nonmatrix presence and dynamic potential. We integrate static (core and wire-line logs) and dynamic (drilling losses and well tests) data sets using a consistent workflow to investigate the types and distribution of nonmatrix excess permeability away from wellbore control. Observations across multiple data sets reveal a multiscale, nonmatrix pore system that is identifiable and important to include in geologic models when considering reservoir performance. The main nonmatrix components include faults and fractures, pervasive networks of touching vugs, centimeter-scale vugs, and caves. Lost circulation zones (LCZ) encountered while drilling through nonmatrix features suggest they are not only open but connected and…

ABSTRACT The North Sea Super Basin is a trilete rift system located in the maritime waters of the United Kingdom, Norway, Denmark, Germany, and the Netherlands. Created after a phase of regional thermal doming in the Middle Jurassic, the rift basin consists of the Viking Graben, Central Graben, and Moray Firth Basin and their surrounding platform areas. Synrift extensional activity occurred during the Upper Jurassic followed by postrift thermal subsidence from the Cretaceous to the present day. The basin’s main Upper Jurassic (Humber or Viking Group) source rocks were deposited contemporaneously with rifting. Their subsequent subsidence history led to progressive maturation, initially focused in the grabens, but becoming ever more extensive with time. Maximum burial of the Upper Jurassic occurs at the present day leading to the efficient charge of a diverse array of overlapping plays. The only exception occurs in western parts of the Moray Firth rift arm, where Cenozoic uplift arrested the maturation of Upper Jurassic source intervals. However, Middle Jurassic (paralic), Lower…

Abstract Extensive, thick, and heterogeneous lacustrine shales occur globally with some regarded as source rocks, but which facies are the key contributors remains unclear. Previous studies have considered that all of the heterogeneous shales were source rocks in the Dongpu depression, but exploration does not support this assumption. Based on 40 yr of geochemical data collection, this study investigated the hydrocarbon generation and expulsion potential of these shales. The results show that laminated shales, though relatively limited in thickness and area, contain much higher organic matter contents and more oil-prone organic matter. Thermal simulation analysis shows that hydrocarbon generation and expulsion potential of laminated shales are much higher and more favorable for hydrocarbon expulsion. Oil-source correlation analyses between oil and shales indicate that northern oils feature pristane/phytane ratios (Pr/Ph) less than 0.7, gammacerane (gammacerane/C30-hopane) indices greater than 0.5, and L-shaped distributions of ααα-C27-20R, ααα-C28-20R, and ααα-C29-20R regular…

ABSTRACT Relative timing of fracturing is a key input for predictive fracture models, but timing information for fractures is commonly difficult to obtain. In this study, we used crosscutting relations and fluid inclusion assemblage temperatures from fracture cements from a few well-documented sampled fractures, combined with a one-dimensional burial history model, to establish timing for three generations of opening-mode fractures in a Barnett Shale core from the southern part of the Delaware Basin, Pecos County, West Texas. A burial history model is presented for the cored well and matched to measured vitrinite reflectance in samples from the core, and bottomhole temperature in the well. The earliest fractures (group 1) likely formed due to early fluid-expulsion events (ca. 300 Ma) and were folded during host-rock compaction. Later group 2 fractures are sealed with fibrous barite containing primary, liquid hydrocarbon inclusions (mean homogenization temperature [Th] = −9°C) and aqueous fluid inclusions (mean Th = 108.1°C). Group 2 fractures likely formed in response to fluid…

ABSTRACT The West Siberian Basin is the largest physical hydrocarbon basin in the world and one of its most prolific. It has proven reserves of 146 billion bbl of oil and more than 1600 TCF of gas. It contains 107 giant fields, 1 mega giant, 10 super giants, and 9 new giants discovered since 2019. Multiple source rocks and reservoirs span Paleozoic through Cretaceous age strata. Structural traps are abundant, many of which have enormous aerial extent. Stratigraphic architecture resulting from multiple transgressions and regressions allows additional combination and stratigraphic trapping. The rich Mesozoic petroleum systems overlie a complex assemblage of deeper Paleozoic basins, Hercynian accreted terranes, and Permian–Triassic rifts. A northern triple junction linked the basin northwestward into the Kara Sea and northeastward to Yenisey-Khatanga trough. The current sag basin developed in the Early Jurassic and persisted until regional uplift in the Oligocene. Strike-slip faults developed along many terrane boundaries, particularly within and near the Triassic rifts, forming…

Abstract The middle Eocene Dongying sag in the Bohai Bay Basin of China has an estimated shale oil resource of approximately 1.1 billion t (8.06 billion bbl); flows of shale oil have been produced in the succession from tens of wells, where the daily production of a single well generally varies between 10 and 100 t (73.3–733 bbl). Therein, the mudrock successions composed of meter-scale mudstone–limestone couplets are the most important shale oil-producing layers. The controls on the deposition of the meter-scale mudstone–limestone couplets, however, remain enigmatic, constraining the analysis of lithofacies and, therefore, sweet spot distributions. Here, we analyze three continuously cored organic-rich successions of mudstone–limestone couplets (371 m [1217 ft] in total) in the middle Eocene Dongying sag, accompanied by decimeter- to meter-scale sampling and testing of mineralogy, organic geochemistry, and paleontology of the rocks. Our integrated cyclostratigraphic analysis shows that the observed mudstone–limestone couplets occur at periods that coincide with Milankovitch…

ABSTRACT The total organic carbon (TOC) content of shale formations is one of the important parameters to characterize the organic richness of unconventional shale reservoirs and identify shale production sweet spots. We have used traditional approaches like the Schmoker density log method and shallow neural networks to estimate TOC from seismic data and well logs. The Schmoker method is easy to use but does not consider any rock constituent changes affecting the bulk density. However, many neural network case studies have blindly fed all of the attributes to networks without understanding physical or statistical relationships between TOC and seismic properties. Therefore, we develop a combined exploratory data analysis for rock and seismic properties and deep learning approach to prevent the overfitting problems and obtain a more reliable TOC estimation model. First, we conduct statistical analysis of seismic properties from well logs and define the influential properties highly correlated with the TOC logs, such as bulk density, P-wave velocity, and Poisson’s ratio. Second, we…