Continental Rift Basin Into A Hydrocarbon Accumulation

Dongying Depression is a Mesozoic-Neozoic terrestrial rift basin with abundant petroleum in the east of China. After 40 years of exploration, the basin has reached medium to intensed exploration stages. The main exploration targets gradually change from structural traps to subtle traps such as the stratigraphic traps, lithologic traps. The exploration became more and more difficult. Now how to improve the exploration efficiency is the key issue in nowadays study.Combined with geochemistry and geology means, first we have a systematic study of source rocks formation, oil generation, migration in both macroscopic and microscopic view, then analyze the source and accumulation process of a series of typical oilfield in different tectonic belts, sum up the relationships between hydrocarbon occurrence and petroleum accumulation, and suggest some favorable targets for future exploration. The following are the main points and views:1. Integrative studies of sequence stratigraphy, lithology association, paleontology and trace elements characteristics suggest the lake basin types are changing during Es4u- ES2l. Es4u and ES3l are dominated by balanced-filled type deposition, while ES3m is dominated by over-filled type sediment. The lake basins evolution is manifested by changes in sedimentary characteristics. Es4U and ES3l consist of a series of transgressive-regressive parasequences with well-developed sedimentary rhythmicity. It indicates the climate-driven lake level fluctuations are common. Excellent source rocks finely laminated and lacking bioturbation are well developed in these two units. It suggests the lake water has relatively persistent stratification and the bottom water is anoxic which inhibit the colonizing of benthic organism. Contrast to the underlying two stratigraphy units, ES3m is predominant by massiveness mudstones and silty mudstones with lower organic abundance, no clear rhythmic characteristics and no lamination or indistinct lamination. These features show that the climate-driven water level is not so strong, the lake water has good circulation and lacks persistent stratification, and the bottom water is venticulated.2. In microscopic scale, excellent source rocks (mainly distributed in Es4U and ES3l) are also well laminated and are dominated by enriched organic matter, which often forms continual organic matter network. Detailed investigations uncover four types of laminae (including carbonate laminae, clay mineral laminae, organic laminaeand pyrite laminae). In contrast, poor to mediate source rocks (mainly distributed in Es3m) are poorly laminated and dominated by disseminated organic matter.3. Source rocks of different units are investigated for their geochemistry difference in organic richness, organic matter type, and biomarker characteristics. Then the criterion for the excellent source rocks is suggested with a total organic carbon of 3.0%. According to the relatively high resistance log characteristics, excellent source rocks from Es4U and ES31 are traced and mapped.4. Based on the organic geochemistry analysis of core samples, three source rocks evolutionary profiles (Es4U, ES31 and Es3m) are set up. It shows source rocks of Es4U deposited in saline to brackish environment generate hydrocarbons in relatively shallow depths than fresh to brackish source rocks of ES31. The differences also manifest by the chemical dynamic parameters and the chemical structure features of the organic matter.5. Surface geochemistry characteristics of organic-mineral complexities and kerogen from different source rocks, including the specific area surfaces, pore structure, surface energy, etc. are investigated for their evolution path with burial.6. According to the natural geochemistry section and natural remnant oil saturation analysis, the effective hydrocarbon expulsion thresholds of different source rocks are determined. The critical depth for Es4U to expel hydrocarbons in large amount is about 2600m, which are corresponding to late abrupt compaction stage, but for ES31 it is at least 3000m, which are corresponding to tight compaction stage. Combined with burial history, surface geochemistry of kerogen and organic-minerals complexities, micro-fabric features and breakthrough pressure experiments, we put forward a lateral migration model for excellent source rocks.7. Synthetically reservoir geology analysis (oil, water and pressure) and oil source correlation indicate that the oils are sourced mainly from the Es4U and ES31 excellent source rocks in Dongying Depression.8. Seven typical oilfields, including Sheng-tuo oilfield, Dong-xin oilfield, Niu-zhuang oilfield, Liang-jia-lou oilfield, Wang-jia-gang oilfield and Ba-mian-he oilfield, which are located in different tectonic belts are investigated for the source-reservoir relationship and accumulation process. The following are the elementary rules: (1) Most of the oils sourced from ES31 are discovered in the sag zone, and the contribution of ES31 decreases from the sag center to the sag margin. In contrast, the contribution of Es4U increases from the sag zone to the slope zone and...