奥陶系岩溶古地貌恢复及对气藏分布的控制作用——以鄂尔多斯盆地苏里格东区41-33区块为例

苏东41-33区块位于鄂尔多斯盆地中央古隆起的北部南缘,长期沉积间断的风化壳不整合面形成复杂的岩溶古地貌,属苏里格气田东部奥陶系碳酸盐岩气藏的重要勘探区。研究基于钻井、测井、地震及生产动态资料分析,综合运用地球物理法、残厚法和印模法,优选本溪组顶9号煤层和马家沟组马五5灰岩为标志层,深入探讨古地貌单元的恢复依据和划分标准,精细恢复岩溶古地貌形态,并明确岩溶古地貌特征及其对气藏分布的影响。研究表明:苏东41-33区块整体处于二级地貌单元岩溶斜坡之上,奥陶系古风化壳上下地层厚度可划分出上薄下薄、上薄下厚、上厚下薄等5种组合类型,细分古坡地、残丘和沟槽3个三级地貌单元;古地貌总体趋势走向为北高南低,出露层位展布由马五4过渡到马五1。中南部发育连片残丘,北部见小型零星残丘,恢复厚度低于15m,残留厚度超过85m;北部发育2条侵蚀沟槽带,充填厚度大于20m,残留厚度小于45m;古坡地分布范围广泛,周围被古残丘与沟槽所分割,残留厚度介于45m^85m之间,恢复厚度大于10m。古地貌类型对风化壳气藏分布具有明显的控制作用,岩溶残丘及残丘边缘带的古坡地储层叠合厚度增大,溶蚀孔洞充填程度低,主力含气层保存完整且连通性较好,是寻找高产井的目标区,而岩溶古沟槽相对不利于天然气富集,所钻遇气井多为低产或无产能。

RESTORATION OF ORDOVICIAN KARST PALEOGEOMORPHOLOGY AND ITS CONTROL ON GAS RESERVOIR DISTRIBUTION:A CASE STUDY FOR BLOCK41-33 IN EASTERN SULIGE AREA,ORDOS BASIN

Block Sudong 41-33is located in the northern and southern margin of the central paleouplift of Ordos Basin.The weathered crust unconformity with long-term discontinuity of deposition forms a complex karst-paleogeomorphology.It is an important exploration area for Ordovician carbonate gas reservoirs in the eastern Sulige gas field.Based on drilling,logging,seismic and production dynamic data,combined with geophysical method,residual thickness method and impression method,the Benxi Formation top 9coal seam and ma55limestone are selected as key layers for discussing the restoration basis and classification criteria of paleogeomorphology units. Then,the Ordovician karst paleogeomorphology was restorated and the characteristics of paleogeomorphology and its influence on gas reservoir distribution were discussed.It shows that the whole area is located on the karst slope of the second-grade geomorphologic unit,the thickness of upper and lower strata of the Ordovician weathering crust can be divided into five types:thin in upper and lower parts;thin in upper and thick in lower parts;thick in upper and thin in lower parts,etc.Three types of third-grade geomorphologic units,palaeoslope,residual hill and groove are further divided.The general trend of paleogeomorphology is high in the north and low in the south,and the distribution of outcrop layer changes from ma54to Ma51.Continuous residual mound developed in central and southern area,while small sporadic residual mound occurs in the north,with recovered thickness less than 15mand with residual thickness more than 85m.In the north area,two erosion channels are developed,with filling thickness greater than 20mand residual thickness less than 45m.The ancient slope land is widely distributed and its surrounding area is divided by ancient residual mound and groove,with residual thickness ranging from 45mto 85 m,and with recovery thickness over 10m.It reveals that the paleogeomorphology plays a significant role in controlling the distribution of weathered crust gas reservoirs.In fact,the reservoir stacking thickness of ancient slope land in karst residual mound and marginal zone of residual mound increased,filling degree of solution pores is low and the main gas-bearing layer with good connectivity is completely preserved,which is the target area for finding high-yield wells.While the karst-ancient groove is not conducive to natural gas enrichment and most drilled wells there proved to be low-yielding or unproductive ones.