Control of facies/potential on hydrocarbon accumulation:a geological model for lacustrine rift basins

The formation and distribution of hydrocarbon accumulations are jointly controlled by ＂stratigraphic facies＂ and ＂fluid potential＂, which can be abbreviated in ＂control of facies/potential on hydrocarbon accumulation＂. Facies and potential control the time-space distribution of hydrocarbon accumulation macroscopically and the petroliferous characteristics of hydrocarbon accumulation microscopically. Tectonic facies and sedimentary facies control the time-space distribution. Lithofacies and petrophysical facies control the petroliferous characteristics. Favorable facies and high porosity and permeability control hydrocarbon accumulation in the lacustrine rift basins in China. Fluid potential is represented by the work required, which comprises the work against gravity, pressure, interfacial energy and kinetic energy. Hydrocarbon migration and accumulation are controlled by the joint action of multiple driving forces, and are characterized by accumulation in the area of low potential. At the structural high, low geopotential energy caused by buoyancy control anticlinal reservoir. The formation oflithological oil pool is controlled by low interfacial energy caused by capillary force. Low compressive energy caused by overpressure and faulting activity control the formation of the faulted- block reservoir. Low geopotential energy of the basin margin caused by buoyancy control stratigraphic reservoir. The statistics of a large number of oil reservoirs show that favorable facies and low potential control hydrocarbon accumulation in the rift basin, where over 85% of the discovered hydrocarbon accumulations are distributed in the trap with favorable facies and low potentials. The case study showed that the prediction of favorable areas by application of the near source-favorable facies-low potential accumulation model correlated well with over 90% of the discovered oil pools＇ distribution of the middle section of the third member of the Shahejie Formation in the Dongying Depression, Bohai Bay Basin.

Hydrocarbon Accumulation in Lacustrine Turbidite in the Rift Basin, Bohai Bay

Three types of turbidites are identified in the studied area, including proximal turbidite along the northern steep slope （alluvial fans, fan delta, and subaqueous fans via short-distance transportation）, distal turbidites along the southern gentle slope （stacked sliding of delta and fan delta front）, and fluxoturbidite in the central depression. Detailed studies of several case histories and the relationship between sedimentary facies and faults suggest a significant role of tectonic setting and faults in the development of turbidite, which created source areas, effected slope topography, controlled the climate and paleo-environment, and formed enough slope angle and slope break for sedimentary instabilities and massive block movement to form turbidite. According to statistics, 0.86%, with an average value of 40.3%, of the trap volumes in the 69 identified Tertiary lacustrine turbidites in the Jiyang Superdepression in the Bohai Bay Basin are filled with oil. The porosity and permeability of turbidite sands vary widely. The productive reservoirs are generally those from the braided channels of both distal and proximal turbidite, and from the main channel of proximal turbidite, with a low carbonate content and the porosity and permeability higher than 12% and 1 mD respectively. Most of the lithologic oil pools in the Jiyang Superdepression are enveloped by the effective source rocks, and the percentage of the trap volume generally increases with the hydrocarbon expulsion intensity of source rocks. This is in contrast with structural-lithologic traps （i.e. proximal turbidite along the steep slope and distal turbidite along the gentle slope）, in which, graben-boundary faults play an important role in oil migration, as the turbidites are not in direct contact with effective hydrocarbon source rocks.

Organic compositions of lacustrine source rocks in Jiyang super-depression and its implication to petroleum geology

The compositions of organic matter in four immature source rocks from Tertiary strata of Jiyang super-depression, the most typical continental rift subsidence basin in East China, have been stud- ied by different extracting methods with CHCl3, MAC and CS2/NMP, respectively. The results suggest that there are great differences among the chemical compositions of organic matter in the source rocks derived from different depositional environments. About 79% of all the organic matter exists by non- covalent bond in the Es4 source rocks which were deposited under the saline lacustrine, indicating that its organic matter is not the real kerogen, but mainly composed of soluble organic matter which is easy to generate hydrocarbon at lower temperature. This is why the immature oils were derived from Es4 source rocks in Dongying depression. In contrast, around 60% of organic matter exists by covalent bond in Es3 source rocks which were deposited under the deep brackish-fresh lacustrine, showing that Es3 source rocks are mainly composed of kerogen producing mature hydrocarbon at higher temperature. The thermal simulation experiments, upon the remaining solid source rocks which were sequentially extracted by the three solvents, have been carried out. The chloroform extracts from the simulation product have been compared with the other three solvent extracts gained at room temperature. It is obvious that re-markable odd/even predominance (OEP) is mainly the characteristic of soluble organic matter; phytane mostly exists in the soluble organic matter by means of non-covalent bonds and characteristics of soluble organic matter are similar to these in immature oils produced in Jiyang super-depression.

Identification and significance of 3β-alkyl steranes in the Eogene lacustrine sediments and petroleum of China

Several homologous series of steranes with alkyl side chains (C1 to C4) at the 3β position have been identified in the Jiyang Eogene lacustrine deposition. It is postulated that its precursors repre sent a new class of steroids, alkylated at the C-3 po sition with a polyhydroxy n-alkane. These precursors may have been formed by the bacterial addition of a ribose sugar to ?2-sterenes, diagenetic alteration products of steroids synthesized by eukaryotes 3-alkyl steroids might substitute for hopanols in bac terial membranes. When they are present in a sam ple, the patterns of the isomer distributions of 3-alky steranes are similar to desmethyl steranes except fo lower rearranged ones. It is shown that the configu rational isomerization of 3-alkyl steranes is trending in line with that of desmethyl steranes with increasing of maturity. The abundance of 3-alkyl steranes may be controlled by the depositional environments; they are primary in saline or near shore lacustrine, mod erate in shallow lacustrine, poor in sub-deep to deep lacustrine relative to the 4-methyl steranes.