古岩溶流域内地表河与地下河成因联系与储层特征——以塔河油田奥陶系岩溶为例

Genesis relation of surface and underground rivers and reservoir charac⁃teristics in paleokarst drainage systems:A case study of Ordovician karst in the Tahe oilfield

在岩溶缝洞储层精细描述过程中,利用钻井校正地震的方法恢复岩溶末期的岩溶地貌,发现塔河油田奥陶系岩溶区内东西两侧发育由走滑断裂形成的地貌高带(分水岭),其间为一向南开口的喇叭口状洼地,南接岩溶盆地;利用钻井、测井与地震综合解释方法,发现洼地北部较陡(坡度为2.9°左右)、南部较缓(坡度为1.5°),发育由众多支流汇聚的2条岩溶水系,西侧一条在北部以地表河为主,在南部转入地下河,东面一条地表河与地下河交替发育,由3段地表河和2段地下河组成。由于强烈的侵蚀与溶蚀作用,形成了不同规模及充填物的5种地表河,但多数地表河内没有发现河流砂岩充填,只有岩溶湖泥灰岩沉积;而地下河溶洞内充填了大量的砂岩和泥岩,成为重要的油气储层。地表河原本是有砂泥沉积的,当下游发育地下河时,洪水把原有的砂泥岩冲入地下河,形成了溶洞砂泥岩沉积,除少数下游地表河残留砂泥岩外,塔河地区奥陶系岩溶地表河基本不存在砂质储层。

The Tahe oilfield lies on a slope of Ordovician karst,where the northern Tarim basin strongly uplifted in early Hercynian time,exposed Ordovician carbonate rock suffered from karstification for 56 Ma,producing complex fracture-cave reservoirs and karst landforms with numerous stone forests and canyons,as well as several karst drainage systems(surface rivers and underground rivers).The objective of this study was to reveal the genesis relationship be⁃tween these surface and underground rivers and define reservoir characteristics in this area.Based on analyses to a large number of data of drilling,clogging,seismic surveys and measurement,the karst landform at the end period of karst development was restored by the method of drilling and seismic correction.And the genesis types and reservoir characteristics of surface rivers and underground rivers were analyzed by rock-minerals analysis and three-dimensional seismic carving technology.The results show that the geomorphologic high zone(watershed)formed by strike-slip faults developed in the east and west sides of the study area,the dustpan karst depression with southward opening lies between them,and a karst basin is present in the south.The northern depression is a steep slope(gradient 2.9°)and the southern is relatively gentle(1.5°).There are two main karst drainage formed by many tributaries developed in the depression,one is in the west that is dominated by surface rivers in the north and turns into underground rivers in the south;the other is on the east side with alternating surface(3 sections)and underground(2 sections)rivers.There are five kinds of surface rivers with different scales and fillings formed by strong erosion and dissolution,and there are no sedimentary sandstone in most surface rivers that are completely filled by the karst lacustrine marl with breccia,but no karstification,and compacted and cemented in the later period,and has almost no reservoir space.However,the under⁃ground river caves and sinkholes were filled a large amount of sandstone and mudstone,which were protected by the cave supporting,and the compaction effect was weak,among them,the porosity of sandstone reach 20%.Research sug⁃gests that the underground rivers are actually the extension of surface rivers below the ground.Originally there were sandstone deposits in surface rivers.Later on when underground rivers developed in the downstream areas,the flood washed the original sand-mudstone into the underground rivers from sinkholes,thus forming sandstone filling in karst caves of underground rivers,resulting in the high-quality oil and gas reservoirs,while almost no sandstone reservoir is present in surface rivers of Ordovician karst in the Tahe oilfield.