酒西坳陷石北次凹油气生成、运移成藏及勘探方向

Petroleum generation,migration and accumulation and exploration potential in the Shibei subsag,Jiuxi depression

酒泉盆地酒西凹陷石北次凹油气勘探历经70余年,仅在古近系发现了白杨河和单北两个小型油田。虽然许多探井在白垩系中也见到良好油气显示,但仅少数探井获得了低产油流,油气勘探始终未获得大突破。该凹陷究竟有没有勘探潜力以及油气勘探方向始终不清楚。通过对该凹陷大量烃源岩样品地球化学分析研究发现,该凹陷发育下白垩统中沟组、下沟组和赤金堡组三套潜在烃源岩。中沟组烃源岩有机质丰度较低、以Ⅲ型有机质为主,处于未成熟阶段,没有生油能力;下沟组和赤金堡组烃源岩有机质丰度高、以II有机质为主,属于优质烃源岩,其中下沟组烃源岩处于未成熟—低成熟阶段,仅具有生成少量低成熟原油的可能性,赤金堡组烃源岩处于低成熟—成熟阶段,具有生成低成熟和成熟石油的能力。石北次凹烃源岩主要生油期为新近纪末期至第四纪早期,目前生烃作用已基本停止。石北次凹已经发现了三种不同类型原油,第一类原油为低成熟原油,分布于浅层古近系,来源于石北次凹低成熟的赤金堡组上段烃源岩;第二类原油为中等成熟原油,分布于西部斜坡浅层古近系,来源于青南次凹下沟组中段烃源岩;第三类原油为成熟原油,分布于凹陷深层赤金堡组砂岩储层,来源于石北次凹成熟的赤金堡组中下段烃源岩。白杨河、单北和单东油田原油属于第二类原油,为长距离运移成藏,鸭儿峡至石北次凹西部斜坡地区为该类原油有利的勘探区域,古近系则为主要勘探目标层系。白南3井、白南6井赤金堡组原油属于第三类原油,为近距离运移自生自储型油藏,石北次凹中北部深层赤金堡组为该类原油最有利的勘探目标层系,包括常规油藏和非常规致密油藏。

The oil and gas exploration in the Shibei subsag of Jiuquan Basin has lasted for more than 70 years,and only two small oil fields,Baiyanghe and Danbei,were discovered in the Paleogene.Although oil and gas shows have been observered in the Cretaceous of many exploratory wells,only low oil product has been obtained in a few exploratory wells,and a breakthrough in oil and gas exploration has never been achieved.Whether the sag has exploration potential and the target of oil and gas exploration remains unclear.Geochemical data of a large number of source rock samples from the sag reveal that the sag developed three sets of potential source rocks in the Lower Cretaceous,namely Zhonggou,Xiagou and Chijinpu formations.The source rocks in the Zhonggou Formation are locally distributed,are poor in organic matter,dominated by typeⅢ kerogen,immature and have no possibility to generate oil.The source rocks in the Xiagou Formation and the Chijinpu Formation contain abundant organic matter,mainly composed of type II kerogen,and are excellent source rocks.The source rocks in the Xiagou Formation are at the stage of thermally immature to low-mature and have only the possibility to generate a small amount of low mature crude oil,while the source rocks in the Chijinpu Formation are mainly in the stage of thermally low-mature to mature and have the capability to generate low mature and mature oil.The oil generation in the sag took place mainly in the late Neogene to early Quaternary and basically stops at present.Three types of crude oil have been discovered in the Shibei subsag.The first type of crude oil is low-mature,distributed in the shallow Paleogene,and is derived from the low-mature source rocks of the upper part of the Chijinpu Formation in the sag.The second type of crude oil is medium mature,distributed in the shallow Paleogene of the western slopes of the sag,and is derived from the source rocks of the Middle Member of Xiagou Formation in the Qingnan subsag.The third type of crude oil is mature,distributed in the deep sandstone reservoirs of the Chijinbao Formation in the sag,and is derived from the mature source rocks of the Middle and Lower members of the Chijinpu Formation in the Shibei subsag.The crude oils in the Baiyanghe,Danbei and Dandong oilfields are attributed to the second type of crude oil,which are migrated into the reservoirs over long distances from the Qingnan sag.The region from the Yaerxia to the western slope of the Shibei subsag is a favorable exploration area for this type of crude oil,and the Paleogene is the main exploration target strata.The crude oils in the Chijinpu Formation of the wells Bainan-3 and Bainan-6 are the third type of crude oil,which are the self-generated and self-stored type of reservoirs with close-range migration.The deep Chijinpu Formation in the north-central part of the Shibei subsag is the most favorable exploration target strata for this type of crude oil,including conventional reservoirs and unconventional tight reservoirs.