Confined Pyrolysis Study on Hydrocarbon Generation Kinetics, Residual Solid and Hydrocarbon Yield of Lacustrine Shale Source Rocks in Bozhong Depression, Offshore Bohai Bay Basin, China

Abstract

There are three intervals of lacustrine shale source rocks (Eocene third member of Shahejie Formation (E2s3), Mid-Oligocene first member of Shahejie Formation (E3s1) and late Oligocene third member of Dongying Formation (E3d3)) in Bozhong Subbasin, Bohai Bay Basin, China. Sealed gold pyrolysis are carried out on rock and kerogen samples of these excellent source rocks to study the hydrocarbon generation kinetics. Solid residue and hydrocarbon yield of the experiment are analyzed to study the TOC and hydrocarbon carbon isotope evolution. Three immature rock samples and three immature kerogen samples from Shanan Depression, and two immature rock samples from Qinnan Depression are pyrolyzed to study liquid hydrocarbon generation. Three mature kerogen samples from Bozhong Depression are pyrolyzed to study gas generation. All immature samples have type I kerogen and TOC of rocks are 2%~5%. Mature samples are type II kerogen and TOC of rocks are 1%~2%. Maceral analysis on polished surfaces of immature whole rocks shows that bright yellow alginite under fluorescent light are laminated and abundant, which indicates sapropelite is dominated in kerogen. Holoclastic rock analysis by X-ray diffraction shows that clay minerals take up from 24% to 48% of all minerals. The others are calcite, quartz, feldspar and pyrite. Each samples are heated in two groups of sealed gold tube (12 tubes each group) under the heating rate of 20 and 2°C/h respectively. Mean hydrocarbon generation activation energy of E2s3, E3s1 and E3d3 immature rock and kerogen samples range from 219 to 226 kJ/mol, and all samples have concentrated distribution. There is no significant difference in kinetics between kerogen and rock samples which indicates that the catalysis of minerals is unobvious. Samples from same depression but different intervals, and samples from different depressions but same interval have similar kinetics, but sample with relative low content of clay minerals and high content of calcite has slightly higher kinetics. Carbon isotope of C14+ liquid products during peak liquid generation can be 11.3% heavier than extracts from unpyrolyzed samples, which must be considered during oil-source correlation. TOC loss of residual solids can reach up to 48%, which must be considered during source rock evaluation. Carbon isotope of methane can be 40% heavier during mature kerogen pyrolysis and the measured carbon isotopes are successfully applied for gas-source correlation.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017