低生烃强度区致密砂岩气形成机制——以鄂尔多斯盆地天环坳陷北段上古生界为例

摘要

As the Upper Paleozoic in the north part of Tianhuan depression in the Ordos Basin,NW China has lower hydrocarbon generation intensity and complex gas-water relationship,the main factors controlling the formation of tight sandstone gas and the distribution of tight sandstone gas in the low hydrocarbon generation intensity area are studied.Through two-dimensional physical simulation experiment of hydrocarbon accumulation,analysis of reservoir micro-pore-throat hydrocarbon system and dissection of typical gas reservoirs,the evaluation models of gas injection pressure,reservoir physical property,and gas generation threshold were established to determine the features of tight gas reservoirs in low hydrocarbon intensity area:(1) at the burial depth of less than 3 000 m,the hydrocarbon generation intensity of (7-10)× 108 m3/km2 is high enough to maintain effective charging;(2) tight sandstone in large scale occurrence is conducive to accumulation of tight gas;(3) differences in reservoir physical property control the distribution of gas pool,for the channel sandstone reservoirs,ones with better physical properties generally concentrate in the middle of sandstone zone and local structural highs;ones with poor physical properties have low gas content generally.Based on the dissection of the gas reservoir in the north Tianhuan depression,the formation of tight gas reservoirs in low hydrocarbon generating intensity area are characterized by "long term continuous charging under hydrocarbon generation pressure,gas accumulation in large scale tight sandstone,pool control by difference in reservoir physical property,and local sweet spot",and the tight gas pools are distributed in discontinuous "sheets" on the plane.This understanding has been proved by expanding exploration of tight sandstone gas in the north Tianhuan depression.%针对鄂尔多斯盆地天环坳陷北段上古生界生烃强度较低、气水分布关系复杂的问题,对低生烃强度区致密砂岩气形成主控因素与分布规律开展研究.通过二维成藏物理模拟实验、储集层微观孔喉含气性系统分析及典型气藏解剖,建立了注气压力、储集层物性、生气下限等综合评价模型,明确了低生烃强度区致密气形成主要特征:①埋深小于3 000 m,生烃强度达(7～10) ×108 m3/km2即可实现有效充注;②致密砂岩层规模性发育有利于聚集致密气;③储集层物性差异控藏,储集层物性较好的砂体,砂带中心区域局部高点富集天然气,而物性较差砂体整体含气,但含气丰度普遍较低.结合天环坳陷北段气藏解剖,提出低生烃强度区“生烃压力长期充注、规模致密砂层聚气、储集层物性差异控藏、局部甜点富集”形成机制及平面呈不连续“片状”分布的规律.天环坳陷北段致密砂岩气拓展勘探实践较好地证实了这一认识.