柴达木盆地西北古近系新近系异常高压形成机制分析

Genetic mechanism of overpressure in the Paleogene and Neogene in the northwestern Qaidam Basin

柴达木盆地西北(以下简称'柴西北区')古近系—新近系异常超压普遍存在。对超压的空间分布特征和形成机制的研究是评价该地区油气成藏条件与资源潜力的关键。依据柴西北区钻井实测压力数据及基于等效深度法计算的地层压力结果,对研究区异常高压的平面和剖面分布特征进行分析。在此基础上,结合柴西北区典型钻井泥岩声波时差曲线特征、不同时期沉积速率、各地层岩性特征、有机质生烃潜力及柴西北地区遭受的构造挤压作用,对研究区超压的形成机制进行分析,并对各控制要素在超压形成过程中的贡献进行定量评价。研究结果表明:柴西北区地层超压起始深度位于大约1 500m,主要发育在古始新统路乐河组—中新统上干柴沟组内,总体上异常高压随深度加深逐渐增大。研究区超压的形成是多种因素综合作用的结果,其中欠压实作用、有机质生烃和构造挤压作用是该区超压形成演化的重要原因。对控制超压形成的各要素定量评价结果显示:欠压实作用是柴西北区超压形成的最重要控制因素,其贡献率高达60%;构造挤压作用次之,其贡献率在20%~30%左右;有机质生烃演化作用对超压形成也有影响,但贡献率相对较小。此外对柴西北区咸湖环境下的盐度分布特征研究初步表明,地层孔隙流体盐度和咸水半咸水环境下沉积的含盐塑性地层对超压的形成和保存也有一定的影响。

Overpressure is considered prevalent in the northwestern Qaidam Basin.Understanding the spatial distribution and genetic mechanism of overpressure is key to the evaluation of hydrocarbon accumulation conditions and resource potential of the region.Here,we analyzed the plane and section distribution characteristics of the area’s abnormal high pressure based on the measured pressure and pressure calculated by the equivalent depth method.Combining the time-difference curve of acoustic wave of mudstone,deposition rate,lithologic characteristics,hydrocarbon generation potential of organic matter and tectonic compression,we quantitatively evaluated the genetic mechanism of overpressure.The results showed that the initial depth of overpressure in the area was about 1500 m,developed mainly in the Lulehe to upper Ganchaigou Formations of the Miocene and with abnormally high pressure increasing with depth.In general,undercompaction,organic matter generation and tectonic compression were important factors for overpressure formation and evolution.The results showed that,in the northwestern Qiandam Basin,undercompaction was the most important controlling factor,with a contribution rate of up to 60%;the contribution from tectonic compression came in second at about 20%-30%;and the contribution from organic matter generation was the least.Moreover,the salinity characteristics of the northwestern Qaidam Basin indicated that the formation of the overpressure was also affected by both pore fluid salinity and the salt-bearing plastic layer deposited in the salt-brackish water environment.