Genetic Mechanism and Quantitative Evaluation of Overpressure in Strike-Slip and Extrusion Zone of the Tan-Lu Fault: Taking JZ27 Section of Liaodongwan in Offshore Bohai Bay Basin, China, as an Example

Abstract

The Tan-Lu fault zone is one of the most important oil and gas enrichment zone in eastern China, its activity is dextral strike-slip since the Neozoic. Because of strong activity from middle Neozoicto to late Neozoic, a strong strike-slip and extrusion stress formed in the prior existent strata. As a result, a local overpressure formed in these pre-existing strata. Surpressure developed in E3s2 (about 36~38Ma) reservoir in JZ27-A wellblock, which is located in Strike-slip and compressor-shear booster zone in JZ27 section of Liaodongwan in Bohai sea area of the Tan-Lu Fault. Average of pressure coefficient is about 1.5. Main characteristics are: 1) thick-bedded and isolated interval of mudstone existed above the overpressure; 2) with heavy cataclasm on grains; 3) Range and horizon are affected by range of booster zone and intense development of Strike-slip and compressor-shear. Genetic mechanism is: on the effect of intense dextrorotatory strike-slip of the Tan-Lu Fault from end of E3d3 (about 31 Ma) to early of E3d2 (about 28 Ma), area of stress concentration has formed in JZ27-A wellblock of strike-slip and compressor-shear booster zone. Thereby, horizontal structural compression stress has formed. At the same time, because of overburden thick-bedded mudrock, stress couldn't be released effective. As a result, overpressure occurred. According to the balance of reservoir pressure, contribution of strike-slip stress to overpressure has been quantitatively evaluated. The analysis result shows that the contribution of strike-slip stress to the overpressure, which occurred earlier than period with large amounts of hydrocarbon generation and expulsion, is from 30% to 35%. Because of existence of thick-bedded and isolated interval of mudstone, the overpressure has been well preserved. It impeded the charging of oil tremendously.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015