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Characteristics of Overpressure Units in a Fault-Block Oilfield with Lacustrine Dolomite Reservoirs in Tanggu Area, Bohaiwan Basin

Abstract

Pore pressure distribution is extraordinary heterogeneous in a fault-block oilfield with lacustrine dolomite reservoirs. This study presents a case study in the Paleogene Sha3-5 member in Bohaiwan Basin, which was characterized as a thicker than 150 m overpressured and highly fractured dolomite reservoir and divided into seven blocks by faults. Measured pressure data in 14 depth intervals of four wells and sonic logs from 12 wells were used in this study. We first analyzed the pore pressure planar and vertical distributions by using isobars and well section correlations. Then we analyzed the mechanism of overpressure generation using burial history and porosity evolution curves. At last, we analyzed the effect of overpressure on oil production by comparing the distribution of overpressure to the oil production and fracture density distribution. Results show that three fault blocks have abnormally high pore pressure with pressure coefficient greater than 1.5; other four fault blocks are with normal pore pressure. The pore pressure system can be described as ring belts with three loops, e.g. inner and middle super-high pressure loops and an outer normal pressure loop. The burial depth of the overpressure surface with coefficient greater than 1.2 is about 2300 m, and the hydrocarbon generating threshold is at about 2600 m. We conclude that the overpressure of Sha3-5 member is due to the combined action of a rapid compaction and a hydrocarbon generation, which both occurred after the rapid filling of Minghuazhen Formation. The heterogeneous distributed structurally fractured zones developed in the dolomite strata were formed in the process of late Paleogene stress field transition and uplifting. The fractured zones are concentrated within the overpressure units and did not affect the distribution of pore pressure but the reservoir quality, e.g. porosity. Wells drilled in the overpressured and highly fractured zones, regarded as ‘sweet spot’, are usually with high-productivity.