Integrated Geochemical and Basin Modeling Approach to Hydrocarbon Systems Analysis, Bohai Basin, Peoples Republic of China

Micah Gardemal, Zvi Sofer, Michael Womer, and Sun Xiao Hong

An integrated hydrocarbon systems evaluation was undertaken in the Bozhong and Liaodong Bay areas of the Bohai Basin in an effort to delineate trends and clarify risks for value-based bid decisions. The Bohai Basin is part of the North China Basin (currently producing >1.2 MMBO/D), which is an intracontinental rift basin 200,000 km² in size and contains up to 11 km of Tertiary continental to lacustrine rocks.

Geochemical analyses of biomarker data (GC/MS) from 43 oils and rock extracts from 41 wells indicate that two main types of lacustrine oils, both generated from the Shahejie 3 Formation, can be distinguished. One type, from the Upper Shahejie 3, has algal affinities high relative gammacerane content, and is generated and expelled at vitrinite reflectance equivalents (VRE) of 0.5 to 0.6 %, based on sterane isomer ratios. The other oil type, from the Lower Shahejie 3, has higher bacterial relative to algal input, lower relative gammacerane content, and is generated and expelled at 0.8 to 0.9 % VRE.

Basin modeling studies and migration pathway analyses were integrated with results from the geochemical work to calibrate the thermal model, estimate timing of generation and expulsion, and calculate potential resources available for migration to traps. A rifting heat flow thermal model was calibrated to measured maturity data (Ro, Tmax, and VRE from sterane ratios) and corrected bottom hole temperature data for 30 wells throughout the basin. The time of major Tertiary rifting was estimated at 55-38 mybp, and beta stretching factors of 1.6 to 2.0 were used. Continuous down-well estimates of hydrocarbon yields for resource estimates were calculated from an empirical relationship between measured TOC and Hydrogen Indices (HI) from well samples and Gardemal log R calculations of TOC and I.

Present day heat flows range from 1.05 to 1.50 HFU with an average of 1.25 HFU over basin deeps. Present day geothermal gradients average 32 to 33 °C/km. Depending on source kinetics, the peak generative window (0.55 to 0.9% Ro) in the basin ranges from 3000m to 4000m. Based on source potential and maturity mapping throughout the basin, timing of peak oil generation from the Shahejie 3 source interval ranges from 26 to 33 mybp. This range of timing suggests that a significant volume of hydrocarbons could have been generated prior to Neogene wrench related structuring and trap formation.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California