Comprehensive Provenance Analysis and Its Applications to Eocene Clastic Rocks in the Huimin Depression, Bohai Bay Basin, China

Clarification of the source of the Eocene sediments filling the Huimin depression provides significant support for current and future oil and gas exploration in the Bohai Bay Basin, China. A comprehensive assessment of sediment provenance based on sandstone petrography, heavy mineral assemblages, seismic reflection data, and distribution of sandstone bodies of the Shahejie Formation (Es2) to the Dongying Formation (Ed) productive interval based on both its macroscopic and microscopic provenance aspects. This method of analysis has the advantages of easy data acquisition, high accuracy, strong flexibility, wide application range, and making the analysis of sedimentary provenances more systematic. Comprehensive analysis of the Huimin depression, Bohai Bay Basin has revealed further that the origin of the Eocene second member of the Shahejie Formation (Es2) to the Dongying Formation (Ed) provenance system was derived from the Linfanjia high, the Chengning uplift, and the Luxi uplift.

Overpressure Generation Mechanisms and Its Distribution in the Paleocene Shahejie Formation in the Linnan Sag, Huimin Depression, Eastern China

The Linnan Sag is one of the main oil-producing units in the Huimin Depression, Eastern China, and the pore pressure gradients obtained from drill stem tests (DSTs) range from 9.0 to 16.0 MPa/km. Uncertainty about the origin and distribution of abnormally high pressures in the Linnan Sag has led to different interpretations of hydrocarbon accumulation and resource assessments, and it interferes with safe drilling. In the Linnan Sag, mudstone compaction curves are substantially affected by several non-compaction factors, and the normal trend of the compaction curve is difficult to determine. The determination of the origin and distribution of overpressure in the Linnan Sag is a challenge. In this study, the factors that may affect mudstone compaction—such as the shale volume, higher calcareous, and organic matter content—were carefully examined and processed. The pressures in the mudstones were estimated by the corrected mudstone compaction curves, which were compiled from acoustic, density, and neutron logs, and calibrated using DST and mud weight data. The log response–vertical effective stress and acoustic velocity-density crossplots were used to identify the mechanisms that generate overpressure. The comprehensive compaction curve shows that the mudstones in the overpressured layer exhibit clear disequilibrium compaction characteristics. The logging response crossplots demonstrate that those overpressured points were consistent with the loading curve. The findings suggest that, the fundamental mechanism resulting in overpressures is the disequilibrium compaction of thick Paleocene mudstones. Hydrocarbon generation and vertical transfer of overpressure may be the main unloading mechanisms, which corresponds to the overpressure points that deviate from the loading curves. Since organic matter cracking may occur in formations at depths greater than 4000 m (Ro > 1.0%), the contribution of hydrocarbon generation to overpressuring is expected to be limited. The transfer of overpressure through opening faults is therefore considered to be the main cause of higher overpressure in local sandstones. The overpressures in the mudstones are characterized by a gradual decrease from the center to the margin in the Linnan Sag. The pressure in the isolated sand bodies are generally similar to that in the surrounding mudstones, whereas it can be lower or higher when the overpressure in the sand bodies are vertically transferred by faults to other pressure systems. The results of this analysis provide an indication of the magnitude, mechanism, and distribution of overpressure in the Linnan Sag. This insight can be used to guide further exploration of the Linnan Sag and similar geological basins.

Application of biomarkers to petroleum migration in the Linyi fault area, Huimin Depression, Bohai Bay Basin, China

A geochemical investigation was carried out on 32 crude oil samples to investigate the origin of the oil and to trace the migration direction in the Linyi fault area, Huimin Depression, Bohai Bay Basin, China. The oils, which were characterized by low gammacerane content and a dominance of C27 over C29 regular steranes, originated from the source rock of the third member of the Shahejie Formation (Es3) in the Linnan Sag. A sequential charge of low- and high-maturity oils from the Es3 source rocks resulted in a decrease in oil maturity in the migration direction. The petroleum migration direction in the footwall of the Linyi fault was northward, away from the fault, and the migration direction at the southwest end (in the hanging wall) of the Linyi fault was westward, along the fault, as evidenced by lateral gradients of biomarker parameters of various maturity, density, and viscosity.

Study on Fault Activity in Linnan Area of Huimin Depression, Bohai Bay Basin

Based on the analysis of growth index, fault throw and fault activity rate, we find the method of fault activity rate integrates advantages of other methods which avoids the influence of erosion and could carry out comparison among fault activities of different geological times. We optimize the method of fault activity rate to analyze the characteristics of fault activities in Linnan area and the results show: the main direction of the faults in Linnan area is nearly NEE-trending and most of the faults have two larger activity periods. It acts intensively in the period of Es2 and Ed and weakens in the period of Es1 and Ng. There is a small comeback in the period of Nm at last. We conduct the force analysis of this area based on stress ellipsoid and find that the stress state agrees well with the regional geological background. The evolution of faults is impacted by the subduction of the Pacific plate and the compression of the Indian Ocean plate.