奥连特盆地斜坡带原油地化特征、充注模式及勘探实践

奥连特盆地斜坡带原油油品分布复杂,重质原油开发难度大、不经济,中轻质原油分布规律不清,严重制约斜坡带油气勘探。本文从原油地化分析入手,通过分析原油分布特征和地化特征,明确重油成因机理,建立原油充注模式,指出中轻质油有利发育区,以有效指导油气勘探。研究结果如下:(1)斜坡带原油正构烷烃具有前峰型碳分布形式,表明原油主要为海相母源;(2)原油具有“双低+双高”族组成,表明原油为烃源岩低熟-成熟阶段产物;(3)重油主要为生物降解成因,证据1为:饱和烃总离子流图发育UCM现象,证据2为:检测发现25-降藿烷标志性化合物;(4)地化分析表明同一样品中同时存在降解原油和未降解原油特征,表明原油为两期充注成因;(5)据此建立油气两期充注模式:早期常规原油充注,中期原油普遍遭受生物降解,原油变重,晚期常规原油充注与降解原油混合控制平面原油油品分布特征,并指出斜坡带砂岩尖灭条带下倾方向是常规油有利分布区,部署探井、评价井获得了成功。

Crude oil geochemical features,charging model and exploration practice in slope zone of the Oriente Basin

The slope zone of the Oriente Basin has a complex crude oil distribution according to °API. Heavy oil extraction in the basin is difficult and costly, accompanied by unclear distribution of medium and light crude oil, which greatly hinders oil and gas exploration in this area. In this paper, starting with geochemical analysis of crude oil, we clarified the formation mechanism of heavy oil and established a charging model for crude oil by analyzing its distribution and geochemical characteristics. We also identified the favorable prospecting areas for medium and light crude oil to provide effective guidance for oil and gas exploration. The results show: (1) Crude oil samples from the slope zone had a front peak in their n-Alkane distribution, indicating marine source. (2) The group composition of crude oil displayed a “double low+double high” feature, suggesting the crude oil comes from the low maturity-maturity stage of source rocks. (3) Biodegradation is the major reason for heavy oil formation, evidenced by (i) the well developed UCM phenomenon in the total carbon ion flow chart; and (ii) the detection of 25-norhopane marker compounds. (4) Degraded and normal crude oil co-exist in the same oil sample according to geochemical analysis, indicating crude oil is the results of mixing two stages of oil charging. Based on the results, a two-stage oil filling model is established: After early oil charging, crude oil is generally degraded and becomes heavier in the mid-stage; then after late-stage oil charging, the mixing of later stage normal oil and degraded crude oil controls the planar distribution of crude oil. The area in the down dip direction of shale belt in the slope zone is the favorable area for normal oil exploration as proved by successful drilling of exploration wells.