鄂尔多斯盆地北缘石炭系-二叠系大型气田形成条件

Forming conditions of large-scale gas fields in Permo-Carboniferous in the northern Ordos Basin

鄂尔多斯盆地北部边缘杭锦旗地区的天然气勘探经历了40年的历史。近10年来,通过在该区进行系统的天然气成藏条件调查与研究,建立了以大型岩性圈闭为主要评价目标的勘探思路,并通过展开勘探取得重大突破,在石炭系-二叠系致密低渗碎屑岩含气层系获得地质储量已超7 000×108m3。综合研究认为,发育在石炭系-二叠系下部海-陆过渡层序中的层状"广布式"近源成藏系统是该区大型气田形成的基本条件。该成藏系统具有大型化的成藏要素:大面积分布的上石炭统煤系源岩、大面积下二叠统河道砂岩储集体、大面积源-储近源配置关系、早白垩世大范围的生烃充注过程及上二叠统大面积区域封盖层。大面积含气、低丰度储量及以岩性气藏为主是杭锦旗地区上古生界气藏的一大特征。在储层普遍致密低渗背景下,寻找富集高产区带、实现有效勘探开发将是面临的挑战。

The Hangjinqi area in the northern margin of the Ordos Basin has been explored for natural gas for about 40 years. However,it wasn＇t until a decade ago that a systematic investigation of gas reservoiring conditions in this area has been initiated. As a result,exploration breakthrough had been made here after aiming at the large-scale lithological traps.The Permo-carboniferous tight clastic rocks were found to contain more than 700 billion m3 of gas-in-place. A comprehensive study shows that large-scale gas fields in the area were mostly formed within the pervasive near-source layered petroleum system in the transitional sequences at the lower Permo-Carboniferous. This petroleum system has widely-distributed coal-measure source rocks in the Upper Carboniferous,huge Lower Permian channel sandstone reservoirs,short distance between the source and the reservoirs,massive hydrocarbon generation and charging during the Early Cretaceous and large area of Upper Permian cap rocks. The Upper Paleozoic gas reservoirs in the area feature in lithological traps,large gas-bearing area and low abundance of reserves. As newly found reservoirs are mostly tight and low-permeability,how to locate sweet spots to realize a highly effective gas discovery is a challenge for all those engaging in petroleum industry.