莺歌海盆地超压体系与油气聚集

OVERPRESSURE SYSTEM AND HYDROCARBON ACCUMULATION IN THE YINGGEHAI BASIN

莺歌海盆地是南海北部被动大陆边缘上的一个新生代沉积盆地，是一个年轻的、快速沉降的、沉积深厚的海相第三纪盆地，具有异常高温、高压，泥底辟构造非常发育。本文用改进了的趋势线法，利用地震速度谱资料计算地层压力，绘制了全盆地的超压顶面等深度图及各层压力系数变化图。经作图发现，莺歌海盆地异常超压体系，象一个漂浮在盆地中的高温高压包，盆地中间埋藏浅而厚，盆地边缘加深变薄以至消失。此外，还发现了古超压面，它与现今的超压面是不一致的。并认为古、今超压面之间是古温度和古压力的能量释放带，现在找到的气田都在这个能量释放带范围内，属于超压体系以上的成油体系。这个成油体系以与泥底辟构造相伴生的高温高压包为烃源岩，由于水热增压和新生流体增压，高压包的压力是不断变化的，当压力增高至冲破上覆地层时，烃类呈幕式从高压包中释放出来，并以水相运移为主，由高势向周围低势区运移。能量释放带从纵向上接受高压包中通过泥底辟和“气烟筒”向上运移的油气。横向上，高压包中的油气沿大断层及不整合面向能量释放带中运移，即向盆地边缘断裂构造带及大三角洲砂岩体内运移、聚集。所以，找准能量释放带中的各类圈闭是油气勘探的关键。

The Yinggehai basin is a Cenozoic sedimentary basin developed on the passive continental margin of the northern part of the South China Sea. It is a young, fast subsiding Tertiary marine basin with thick sediments, where abnormal high temperatures and high pressures (HTHP) have been encountered and mud diapirs were well developed. Presented in this paper are isopach maps of the tops of abnormal overpressure and charts of the transformation of pressure coefficients of horizons of the whole basin, which were derived by using improved trend lines and formation pressures converted from seismic velocity. It was discovered through mapping that the abnormal overpressure system in Yinggehai basin is in fact a HTHP packet floating in the basin. It can be seen from the isopach maps of the tops of abnormal overpressure that it is shallower and thicker in the central part of the basin, and deeper and thinner towards the margin of the basin before eventually disappearing. Additionally, a paleo-overpressure interval has been discovered which is inconsistentwith the present day one. We believe that the interval between the paleo and present overpressure intervals was an energy release zone of paleogeotherm and paleopressure. All of the gas fields recently discovered in the basin occur within this zone, belonging to the petroleum system above the overpressure system. (The petroleum system above the HTHP packet in Yinggehai basin is called the upper petroleum system, below it called the lower petroleum system). The HTHP packet with its associated mud diapirs contain the source rocks for the upper petroleum system. The pressure of the HTHP packet changed continuously with the hydrothermal and young fluid supercharges. When the pressure increased and broke the overlying formations, hydrocarbons were released intermittently from the HTHP packet and migrated mainly in water phase from a high potential field towards the surrounding low potential field. The energy release zone allowed hydrocarbons to migrate vertically along mud diapirs and gas chimneys fromothe HTHP packet. Horizontally, hydrocarbons migrated from the HTHP packet into the energy release zone through large faults and unconformities , i. e. migrated into and accumulated in fault-structural zones along the margin of the basin and into large deltaic sand bodies. Therefore, it is important to accurately identify traps within the energy release zone which is would accumulate hydrocarbon.