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Using Quantitative Characterization of Strike-Slip Restraining Bends to Predict Hydrocarbon Accumulation—A Case Study From Liaodong Bay Segment, Tan-Lu Fault Zone, East China


One of the remarkable tectonic features of strike-slip fault system is the widespread presence of paired bends, which means alternating areas of localized restraining and releasing bends along the principal displacement zone. The same phenomenon is observed at Liaodong Bay segment of Tan-Lu fault zone in East China. Exploration results show that the implication of hydrocarbon accumulation between restraining and releasing bends are pronounced different at Liaodong Bay area. Many large hydrocarbon traps were found along restraining bends, in addition, the faults that control traps in restraining bends also have better capacity of sealing. Although qualitative studies are widely developed, there are limited quantitative studies on strike-slip restraining bends impact on hydrocarbon accumulation. In this study, a quantitative characterization of restraining bends was conducted. It’s observed that hydrocarbon accumulation are strongly correlated with the curvature of master strike-slip faults and intensity of adjusting strike-slip faults occurrence.Based on well-logs and seismic data, the concept of "oil and gas enrichment index" (OGEI) is defined as the product of master strike-slip fault curvature and the intensity of adjusting strike-slip faults. A mathematical model based on OGEI is developed to quantitatively predict the oil and gas enrichment characteristics of various formation from Neocene, Oligocene and Paleocene - Eocene. Three main results are following. First, the minimum OGEI cutoffs for existing traps in different formation (Neocene, Oligocene and Paleocene - Eocene) are 1, 4 and 6 individually; traps with OGEI below the lower limit are difficult to have commercial hydrocarbon accumulation. Second, the curvature of master strike-slip fault and intensity of adjusting strike-slip faults are dynamically coupled with strong positive correlation; this is because the intensity of strike-slip faults is proportion to the tectonic stress concentration which is directly indicated by the density of adjusting strike-slip faults and intensity of individual regulating fault. Third, based on drilling outcomes, OGEI has strong positive correlation with commercial hydrocarbon discoveries which verifies that OGEI is an effective indicator of hydrocarbon accumulation.In summary, OGEI provides an effective and straight-forward measurement of hydrocarbon accumulation, which proven to be commercially successful in Liaodong Bay offshore oil and gas exploration.