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 subbasin of western Texas, SHmax is consistently approximately east–west and normal–strike-slip faulting is observed. Farther west, the Delaware subbasin of western Texas and southeastern New Mexico is characterized by normal faulting and SHmax rotates ∼150° clockwise from north to south. Marked changes in SHmax direction also occur across the Raton Basin of southern Colorado and northern New Mexico, the Denver-Julesburg Basin in northern Colorado, and the Uinta Basin in northeastern Utah, likely associated with their location near the margins of extensional provinces. The new data sets we present help improve operational efficiency by constraining absolute stress magnitudes and the ideal azimuth to drill horizontal wells (i.e., perpendicular to the local SHmax orientation) and make it possible to predict which fractures and faults are likely to be activated during hydraulic stimulation.

ABSTRACT Concretions are common features in organic-rich mudstone successions, where they appear aligned along certain stratigraphic surfaces. Their extreme hardness and often large dimensions make them an unwelcome drilling and fracturing hazard in unconventional reservoirs. This study quantifies the number of calcite concretions along stratigraphic horizons in outcrops of the Vaca Muerta Formation of the Neuquén Basin, Argentina, with the aim of predicting the occurrence in the same horizons in the subsurface. We measured thickness, length, and spacing of more than 800 concretions from 46 different stratigraphic levels of Tithonian and early Berriasian age. Data were gathered along scanlines of variable length in the Sierra de la Vaca Muerta, and Puerta Curaco and Pampa Tril areas. Concretions display significant lateral and vertical variability. Size, spacing, and density index tend to increase toward cycle tops, concomitant with the occurrence of more elongated concretions. In time-equivalent levels at different outcrop locations, larger and more elongated concretions are observed in more proximal settings. In addition to sedimentation rates, an important controlling factor for size and shape of concretions is the availability of calcium carbonate that varies within the sequence-stratigraphic framework and between different areas of the basin. This comprehensive quantitative analysis of concretion occurrence within the Vaca Muerta Formation can help in identifying target intervals less affected by concretion occurrence, and in building more reliable risk-mitigation strategies for drilling and fracturing.

ABSTRACT Continuous petroleum accumulations are becoming important in petroleum exploration. This paper reports a new giant oil play within a lacustrine stratum, the Lower Triassic Baikouquan Formation of the northwestern Junggar Basin, northwestern China. This giant oil play is typical of a continuous petroleum accumulation, with low reservoir porosity and permeability (averages of 7.52% and 0.97 md, respectively), continuous hydrocarbon distribution over a large area (∼1000 km2), stratigraphic traps, and anomalously high formation pressure (pressure coefficient >1.1). Detailed molecular as well as isotope geochemistry of hydrocarbons reveal that crude oil and natural gas in the Baikouquan oil play were derived predominantly from the alkaline lacustrine source rock of the lower Permian Fengcheng Formation. Dolomitic source rock is the main contributor to the crude oil and was deposited under anoxic, saline to hypersaline conditions. The analysis of spatial difference of oil maturity in combination with stratigraphic burial-thermal evolution indicates multiple charging events and vertical migration as the dominant process for oil charging in the Baikouquan play. As such, this giant Baikouquan oil play is interpreted to differ from classic continuous petroleum accumulation because of long-distance (1500–2500 m) vertical hydrocarbon migration from deeper Permian sources and is thus referred to as a somewhat atypical continuous oil play in this study. A strike-slip fault network provides migration pathways for the vertical oil charging, and a fan–delta-front conglomerate facies provides high-quality reservoir rocks. The discovery of the Baikouquan oil play exemplifies the successful exploration of a large-area continuous oil accumulation at a long distance (kilometers) from the pod of active source rock, and highlights new exploration opportunities worldwide within continuous oil accumulations, where the slope and central zones of a sag might represent viable petroleum reservoir rock despite their distance from a mature source rock.

ABSTRACT Recent discoveries of huge tight oil resources in the middle Permian Lucaogou Formation within the Jimusaer sag in the Junggar Basin in northwestern China have aroused numerous studies on this unconventional petroleum system. However, depositional conditions of the source rocks and the accumulation models of the oils are still unclear because of the complexity of the petroleum system. In view of this, systematic geochemical investigations on 120 samples covering a whole variety of source and reservoir rocks in the Lucaogou Formation were carried out. Based on the results, two types of depositional conditions were identified for the five sections of source rocks (denoted A–E), with sections A, C, and D deposited under hypersaline and reducing conditions, and sections B and E under brackish and relatively oxidizing conditions. Oil-source rock correlation results demonstrated that four groups of oils (denoted 1–4) with distinctive geochemical signatures from the formation finally accumulated in three relatively independent subsystems (labeled Sub-I to Sub-III). Groups 2–4 oils (Sub-II and Sub-III) originated solely from interbedded mudstone, so the accumulation model is highly interbedded and nearby accumulation. Group 1 oils (Sub-I) are originated from the vertical mixing of hydrocarbons produced by multiple mudstone beds at greater depths followed by lateral migration, thus called vertical-lateral mixing accumulation model. This study highlights the importance of lateral migration in tight oil accumulation and will benefit future tight oil exploration in the Junggar Basin, such as the Permian Feng Cheng Formation, and other areas worldwide.

The interpretation of the depositional environment, provenance, and paleoclimate provides important information for understanding the formation and distribution petroleum systems. The trace element and rare earth element (REE) concentrations and the heavy mineral distribution in upper Paleozoic sedimentary rocks in the Linxing area, northeastern Ordos Basin, were determined to reconstruct the paleoenvironment. The samples show clear light REE enrichment, even though the degree of enrichment is generally poor. Redox proxies indicate that most of the samples were deposited in relatively dysoxic-oxic conditions. A warm and humid climate prevailed during deposition of the Benxi to Xiashihezi Formations, while extremely dry climate also occurred during Benxi Formation and Taiyuan Formation deposition. Shiqianfeng Formation deposition occurred in a completely dry climate. The increase in ΣREE concentrations and the decrease in the Sr/Ba ratios above the Benxi Formation indicate a rapid increase in terrigenous clastic input and sea-level fall, which was related to Permian tectonic movement. Bivariate plot diagrams show mixed felsic-basic source influences for all the formations, with the Shanxi Formation displaying certain influences from acidic sources. From late Carboniferous to early Permian, the Daqingshan uplift was the primary provenance. Subsequently, the Yinshan paleocontinent played a more important role as the main provenance as it was being continuously uplifted.

ABSTRACT The Carter-Knox field is located on a northwest-southeast–trending faulted anticline in the southeastern part of the Anadarko Basin. Three-dimensional seismic and well data were used to develop a model for the geometry and evolution of the structure. The Carter-Knox structure formed during contractional deformation associated with the Wichita basement uplift in the Pennsylvanian. It is characterized by different structural styles in two main structural units. The lower unit, which includes the Cambrian Arbuckle Group to Mississippian Sycamore Limestone, is folded into a broad anticline associated with one or more frontal faults and back thrusts, with a change in vergence along trend. The upper unit, which includes the Upper Mississippian Springer shale to the Morrowan Primrose and the overlying Pennsylvanian growth units, is marked by a tight faulted detachment fold with a steep front limb, associated with multiple thrust faults that detach within the Springer shale. Kinematic reconstruction shows differential shortening between the two main structural packages. The evolution of the structure was episodic, resulting in two major angular unconformities within the Pennsylvanian. A major Permian unconformity truncates the structure. The southwestern flank of the structure was rotated along an active flexural hinge in the basement because of sedimentary loading, resulting in thickening of growth units on this flank. The Carter-Knox structure is an analogue for other thin-skinned structures in the Anadarko Basin and illustrates the impact of the thick-skinned Wichita uplift on the thin-skinned fold-thrust structures in the basin.