临兴地区致密砂岩气藏形成机理与成藏模式

本文以鄂尔多斯盆地临兴地区上古生界致密砂岩气成藏机理与成藏模式为研究目标,通过测井资料解释、流体包裹体测定和古流体压力恢复等方法,总结了气藏类型与分布特征,分析了其成藏期次、运移动力和输导体系,阐明了形成机理与成藏模式。研究表明,临兴地区上古生界主要含气层位为太原组、山西组和下石盒子组,气藏分布受生储盖组合、构造部位和储层物性控制。临兴地区上古生界主要有2期成藏,中侏罗世为第1期,天然气以CO2为主、含少量气态烃,储层中流体剩余压力较低,裂缝多未切穿石英颗粒,天然气运移速率较低,汇聚成藏规模小;早白垩世为第2期成藏,天然气以甲烷为主、含少量重烃和液态烃,储层中剩余压力较高,裂缝普遍发育并切穿石英颗粒,与断层结合形成了三维网状输导空间网络,流体运移速率大,运移距离长,汇聚面积大,形成了一系列大型气藏,是临兴地区上古生界最主要的成藏期。靠近紫金山岩体的地层,在晚侏罗世到早白垩世期间遭受高压流体的侵入,形成一个以岩体为中心向外逐渐减弱的成藏作用带。临兴地区致密砂岩天然气成藏模式可总结为:“两期成藏、晚期为主;早期非烃,晚期甲烷;超压为主、浮力次之;孔缝为主、断裂为辅;岩性占优、构造偏少”。

Formation mechanism and model of tight sandstone gas reservoirs in the Linxing area of Ordos Basin

The purpose of this paper is to study the generation mechanism and model of gas reservoirs in tight sandstone of Upper Paleozoic in the Linxing area, Ordos Basin. Through logging data interpretation, fluid inclusion measurement and paleofluid pressure recovery, the types and distribution characteristics of gas reservoirs are summarized, the accumulation periods, migration dynamics and transport system are analyzed, and the formation mechanism and model of the reservoirs are clarified for this area. The study shows that the main gas-bearing strata of the Upper Paleozoic in the Linxing area are Taiyuan Formation, Shanxi Formation and Lower Shihezi Formation, and the distribution of gas reservoirs is controlled by source-reservoir-cap combination, structural location and reservoir physical properties. The Upper Paleozoic experienced two stages of accumulation. Of them, the Middle Jurassic is the first stage, during which the natural gas was dominated by CO2 with a small amount of gaseous hydrocarbon, the residual pressure of fluid in the reservoir was low, fractures did not cut through quartz particles, natural gas migration rate was low, and the accumulation scale was small. The early Cretaceous is the second stage of hydrocarbon accumulation, where natural gas was mainly methane with a small amount of heavy hydrocarbon and liquid hydrocarbon, residual pressure in reservoirs was relatively high, and cracks developed well cutting through quartz grains. Combined with faults, these cracks formed a three-dimensional network of transport, through which fluid migration rate was high, migration distance was long, and the convergence area was large. Consequently a series of large gas reservoirs were formed, which were the most important gas accumulation period in the Upper Paleozoic of the Linxing area. The strata close to the Zijinshan pluton were invaded by high-pressure fluid from late Jurassic to early Cretaceous, forming a gradually weakened reservoir forming zone centered by rock mass. The reservoir forming model of tight sandstone gas in the Linxing area can be summarized as follows: It is characterized by two stages of hydrocarbon accumulation, of which the late stage is main accumulation period. The non-hydrocarbon gas of early natural gas is dominant and the methane proportion is relatively high in late natural gas. The overpressure, followed by buoyancy, plays a critical role in gas migration. The pores and cracks are dominant in gas migration pathway and faults are auxiliary. The lithology gas pool is dominant and the structure gas pool is less.