鄂尔多斯盆地北部构造叠置与天然气聚集研究

Tectonic superposition and accumulation of natural gas in northern Ordos Basin

鄂尔多斯盆地北部的构造-沉积演化相比盆地主体具有特殊性和复杂性,尽管勘探已发现了多套气层,但不同区带多层天然气差异成藏规律仍不清楚.本文利用伊陕斜坡—伊盟隆起构造过渡带上杭锦旗地区井、震资料,结合区域构造演化研究,恢复了中元古界—古生界残存地层分布,分析了中元古代以来6个关键时期的构造、沉积演化过程.根据元古宇与古生界的构造叠置关系,将研究区划分为断陷-斜坡、断陷-凸起、凸起-断阶和凹陷-斜坡4种类型,认为构造演化控制了1套主力烃源岩和3套勘探层系储层的差异叠合:石炭-二叠系煤层烃源岩成熟度呈西高东低,西部镜质体反射率R为1.3%~1.7%,东部镜质体反射率R为0.8%~1.3%;二叠系下石盒子组储层物性呈东好西差,东部孔隙度φ为10%~20%,西部孔隙度φ为5%~10%;奥陶系马家沟组及中元古界长城系储层在局部地区残存分布.研究结果表明:在源-储差异配置与构造演化控制下,断陷-斜坡叠置区主要发育二叠系地层-岩性气藏及长城系潜山气藏,是寻找多层富集目标的最有利区带;凹陷-斜坡叠置区是奥陶系灰岩缝洞型气藏与二叠系构造-岩性复合气藏叠合发育有利区;凸起-断阶叠置区是上古生界多类型气藏复合发育区;断陷-凸起叠置区则是长城系及二叠系源外气藏潜在发育区,可作为近期勘探突破目标.石炭-二叠系高成熟烃源岩(R>1.3%)分布控制了3套层系天然气叠合富集,整体为源内充注成藏模式;高成熟烃源岩区之外主要为源侧运移成藏模式,继承性古隆起上的有效圈闭是天然气聚集的主控因素.

Compared with the central area of Ordos Basin, the tectonic and depositional evolution of the northern margin of Ordos Basin is more special and complex. Although several gas layers have been found in exploration, the differential accumulation rules of multi-layer natural gas in different zones are still unclear. Based on the analysis of well and seismic data in the Hangjinqi area from structural transition zone between the Yishan slope and the Yimeng uplift, combined with the study of regional tectonic evolution, the distribution of residual strata from Middle Proterozoic to Paleozoic is restored, and the tectonic and depositional evolution of six key periods since the Mesoproterozoic are rebulit. According to the analysis of the tectonic superposition relationship between the Proterozoic strata and the Paleozoic strata, the tectonic superposition relationship in the Hangjinqi area can be divided into four types: fault depression-slope, fault depression-uplift, uplift fault-terrace and depression-slope. It is considered that the tectonic evolution controls the differential superposition between one main source rock and three major reservoirs. The maturity of source rocks in Carboniferous and Permian coal seams is high in the west and low in the east, and the vitrinite reflectance(R)of the west and east area is 1.3%—1.7% and 0.8%—1.3%,respectivly. The physical properties of the Lower Shihezi formation of Permian are good in the east and poor in the west, and the porosity(φ)of the west and east area is 10%—20% and 5%—10%,respectivly. The Majiagou formation of Ordovician and Middle Upper Proterozoic Changcheng reservoir are distributed residually in some areas. The results show that, under the control of differential configuration of source rock-reservoir and tectonic evolution, Permian stratigraphic lithologic gas reservoirs and Changcheng buried-hill gas reservoirs are mainly developed in the fault depression slope superposition area, which are the most favorable zones of multi-gas layer enrichment. The tectonic superimposed area of depression and slope is favorable for the superimposed development of Ordovician limestone fracture cave gas reservoir and Permian sandstone structural lithologic composite gas reservoir. The tectonic superimposed area of uplift and fault terrace is the compound development area of multiple types of gas reservoirs in Upper Paleozoic. The tectonic superimposed area of fault depression and uplift zone is a potential development area of Changcheng and Permian gas reservoirs, which could be a breakthrough exploration target in the near future. The distribution of the Carboniferous and Permian high mature source rocks(R>1.3%)controlled the enrichment of three superimposed gas accumulations, which is characterized by in-source charging and accumulation mode. By contrast, the area outside the high mature source rocks is distinguished by the out-source migration and accumulation mode. The effective traps on the inherited paleo-uplift is the main controlling factor of natural gas accumulation.