Hydrocarbon source rock assessment of the shale and coal bearing horizons of the Early Paleocene Hangu Formation in Kala-Chitta Range, Northwest Pakistan

The present study aims to investigate the origin, type, thermal maturity and hydrocarbon generation potential of organic matter and paleo-depositional environment of the Early Paleocene (Danian) Hangu Formation outcropped in the Kala-Chitta Range of Northwest Pakistan, Eastern Tethys. Organic-rich shale and coal intervals were utilized for geochemical analyses including TOC (total organic carbon) and Rock–Eval pyrolysis coupled with carbon (δ13Corg) and nitrogen (δ15Norg) stable isotopes. The organic geochemical results showed that the kerogen Type II (oil/gas prone) and Type III (gas prone) dominate the investigated rock units. The TOC (wt%) and S2 yield indicate that the rock unit quantifies sufficient organic matter (OM) to act as potential source rock. However, the thermal maturity Tmax°C marks the over maturation of the OM, which may be possibly linked with the effect attained from nearby tectonically active Himalayan Foreland Fold-and-Thrust Belt system and associated metamorphosed sequences. The organic geochemical analyses deciphered indigenous nature of the OM and resultant hydrocarbons. The δ13Corg and δ15Norg stable isotopic signatures illustrated enrichment of the OM from both marine and terrestrial sources accumulated into the Hangu Formation. The Paleo-depositional model established using organic geochemical and stable isotopic data for the formation supports its deposition in a shallow marine proximal inner shelf environment with prevalence of sub-oxic to anoxic conditions, a scenario that could enhance the OM preservation. Overall, the formation holds promising coal and shale intervals in terms of organic richness, but due to relatively over thermal maturation, it cannot act as an effective source rock for liquid hydrocarbon generation and only minor amount of dry gas can be expected. In implication, the results of this study suggest least prospects of liquid hydrocarbon generation potential within Hangu Formation at studied sections.

Particle-Size Fractionation and Thermal Variation of Oil Shales in the Songliao Basin, NE China: Implication for Hydrocarbon-Generated Process

The synchronous variation and association of organic matter (OM) and minerals in the hydrocarbon-generated process of oil shales are poorly understood. The goal of the paper is to investigate OM occurrence and thermal variation so as to reveal the hydrocarbon generation potential of oil shales. Based on detailed analyses of particle, organic, mineral, and thermal data from lacustrine oil shales in the Songliao Basin, we observed three layers of shale particles after settling in the water column characterized by a distinct color, degree of consolidation, and particle size. The particle sizes are divided into three ranges of fine grain (<1 μm), medium grain (1–20 μm), and coarse grain (>20 μm) via laser particle analysis. The particle-size distribution indicates the presence of OM polymerization and dominant contribution of the associated mineral surface and bioclastic OMs to the OM abundance of oil shale. Various OM occurrences are influenced by OM sources and redox conditions, whereas the degree of biodecomposition and particle sizes affect the placement of OM occurrences. Based on multiple thermal analyses, a synchronous response of OM and minerals to thermal variation dominates at 300–550 °C. The I/S and chlorite minerals are characterized by an entire illitization, while solid/absorbed OMs and hydrocarbon-generated water were expelled in large quantities. This contributes to major loss weights of oil shales during heating. The peak hydrocarbon-generated rate occurred at 457 °C for oil shales, corresponding to around 1.3% vitrinite reflectance value. These results are suggested to improve the understanding of OM occurrences and the thermal degradation constraint on the hydrocarbon-generated process, and contribute to the interpretation of the hydrocarbon generation potential and in-situ exploitation of oil shales.

Identification of overpressures resulting from undercompaction and hydrocarbon generation in shale dominated settings using well log data

Based on the concepts of organic geochemistry and well log characteristics, we discuss the differences between undercompaction overpressure (UCOP) and hydro-carbon-generating pressurization (HGP) from well logs in the Chang 7 Shale, Ordos Basin. The results revealed that shales have better hydrocarbon generation potential than mudstones. We found the UCOP is characterized by significant changes of acoustic traveltime (AC), compensation neutron (CNL), density (DEN) and resistivity (RT) on the well logs. Whereas the HGP causes obvious changes in the AC and RT logs, minor deviations are generally revealed in the CNL and DEN logs. Further stud-ies revealed that while UCOP occurs primarily in thick shale layers of the sedimentary center, HGP is predominant in the thin shales of the sedimentary edge. The reason for the above difference may be attributed to the ubiquity of the shales associated with HGP, with the variation in Well logs caused by the HGP concealed by the abnormal variation of UCOP. These findings thus revealed the distribution and well log features of abnormal overpressure for the different generating mechanisms, thus providing significant guidance for further exploration and development of the overpressured formations.

Revisiting Controls on Shale Oil Accumulation in Saline Lacustrine Basins: The Permian Lucaogou Formation Mixed Rocks, Junggar Basin

Fine-grained mixed rocks in saline lacustrine basins are important targets for shale oil exploration. However, the controls on shale oil accumulation are complex due to the multi-source mixed deposition. This is a challenging issue in the study of shale oil. Here, we present a case study in the Middle Permian Lucaogou Formation in the Jimusar Sag, Junggar Basin, northwestern China. Results show that the Lucaogou Formation consists mainly of carbonate rocks, dolomitic or calcareous mudstones, tuffaceous or silty mudstones, and siltstones. The dolomitic/calcareous mudstones ( TO C average = 6.44 wt . % ) and tuffaceous/silty mudstones ( TO C average = 4.83 wt . % ) have the best hydrocarbon generation potential and contain type I–II1 kerogens that are in the peak oil generation stage. However, the shale oil potential is highest for the carbonate rocks and siltstones with average oil saturation index (OSI) values of 315.03 mg HC/g TOC and 343.27 mg HC/g TOC, respectively. This indicates that hydrocarbon generation potential is not the main factor controlling shale oil potential. Micro-nanoscale pores are the main control. Abundant dissolution pores provide excellent reservoir space for near-source migration and accumulation of shale oil. Different mixing processes between lithofacies control the accumulation of shale oil, and shale oil productivity is the best when multi-facies deposition in transitional zones formed the mixed rocks (facies mixing). In addition, local accumulations of calcareous organisms and adjacent carbonate components on terrigenous sediments (in situ mixing) are also conducive to shale oil enrichment. This is an unusual and special feature of saline lacustrine shale oils, which is different from freshwater lacustrine and marine shale oils. Comprehensive assessment of source rock and reservoir is needed to robustly establish a widely applicable method to determine the shale oil potential in such basins.

Thrusts control the thermal maturity of accreted sediments

Thermal maturity assessments of hydrocarbon-generation potential and thermal history rarely consider how structures developing during subduction influence the trajectories of accreted sediments. Our thermomechanical models of subduction support that thrusts evolving under variable sedimentation rates and décollement strengths fundamentally influence the trajectory, temperature, and thermal maturity of accreting sediments. This is particularly true for the frontal thrust, which pervasively partitions sediments along a low and a high maturity path. Notably, our findings imply that interpretations of the distribution of thermal maturity cannot be detached from accounts of the length and frequency of thrusts and their controlling factors. Taking these factors into consideration, our approach reduces former inconsistencies between predicted and factual thermal maturity distributions in accretionary wedges and provides a first-order predictive indicator for thermal maturity distribution based on known fault architectures.

Paleovegetational Reconstruction and Implications on Formation of Oil Shale and Coal in the Lower Cretaceous Laoheishan Basin (NE China): Evidence from Palynology and Terpenoid Biomarkers

In some cases, the oil shale deposited in shallow lakes may be genetically associated with the coal-bearing successions. Although paleovegetation is an important controlling factor for the formation of oil shale- and coal-bearing successions, few studies have focused on their joint characterization. In this study, a total of twenty-one oil shale and coal samples were collected from the upper member of the Lower Cretaceous Muling Formation (K1ml2) in the Laoheishan Basin, and investigated for their bulk geochemical, maceral, palynological, and terpenoid biomarker characteristics, in order to reconstruct the paleovegetation and reveal its influence on the formation of oil shale and coal. The K1ml2 is subdivided into lower, middle, and upper units. The studied oil shale samples from the lower and upper units display a high ash yield (Ad), low total organic carbon (TOC) and sulfur (S) contents, and limited hydrocarbon generation potential. The studied coal samples from the middle unit are characterized by low Ad, and high TOC and low S values, and show significant hydrocarbon generation potential. The paleovegetation during the formation of the lower unit was dominated by mire vegetation, such as shrubs (e.g., Lygodiaceae, Schizaeaceae), tree ferns (e.g., Dicksoniaceae/Cyatheaceae), and coniferous trees (e.g., Podocarpaceae). In the middle unit interval, the paleovegetation was represented by highland vegetation (Pinaceae and Araucariaceae) and peat-forming coniferous plants (e.g., Podocarpaceae, Cupressaceae/Taxodiaceae). Various vegetation, such as herbs (e.g., Osmundaceae), shrubs (e.g., Schizaeaceae), and coniferous trees (e.g., Podocarpaceae) was prosperous during the upper unit interval. Coniferous trees could provide abundant hydrogen-rich materials (e.g., resins) to the mire/lake, which may elevate the hydrogen content in peat/lake sediments, and finally result in higher hydrocarbon generation potential in the coal than in the oil shale. Therefore, the influence of paleovegetation on the formation of oil shale and coal should be fully considered when studying oil shale- and coal-bearing successions. The results also provide guidance for further exploration studies on oil shale and coal in northeast China.