珠江口盆地恩平凹陷CO_(2)成藏特征与成藏过程

Characteristics and Processes of CO_(2) Accumulation in the Enping Sag,Pearl River Mouth Basin

珠江口盆地恩平凹陷北部潜山披覆带首次发现了规模富集的CO_2气层,但分布局限,平面上主要分布在A油田1井区,纵向上主要分布在珠江组上段,其次是珠海组,其他层系和构造高点未见CO_2富集。CO_2气藏含量普遍在92%以上,δ~(13)C为-8.735‰~-3.024‰,~3He/~4He介于5.75×10~(-6)~8.16×10~(-6)之间,为典型的幔源火山成因。由于A油田周边数十千米内无火山发育,且CO_2含量从深到浅逐渐降低,而位于A油田南侧的F_4断裂是在先存基底断裂基础上新生代持续活动的断裂,向下切入基底数千米,向上断至粤海组,活动强度大,故推测F_4深大断裂是A油田CO_2气藏的气源断裂。A油田油气成藏过程具有"两种原油类型、两期原油充注、一期CO_2驱油远藏"的特点,CO_2充注驱油增强了油气二次运移的动力,促进了油气的长距离运聚,扩大了恩平凹陷乃至整个珠江口盆地北部断阶带的勘探层系和勘探边界。

A large-scale enrichment of CO2 gas reservoir has been found for the first time in the buried hills of the northern onlap zone in the Enping Sag, but its distribution is limited. It is only enriched in the Well 1 block of Oilfield A. CO2 gas reservoir is mainly distributed in the upper member of the Zhujiang Formation, and less in the Zhuhai Formation. No CO2 enrichment is found in other strata and other structural highs. The δ13 C values of the Oilfield A range from-8.735‰ to-3.024‰, which is of inorganic origin;3 He/4 He ratios range from 5.75×10-6 to 8.16×10-6, which are typical for volcanic mantle-derived CO2. No volcanic activity was reported within tens of kilometers around Oilfield A in the Enping Sag, and the CO2 contents gradually decrease from deep to shallow. The fault F4 located in the southern Oilfield A is a Cenozoic continuous active fault on the basis of the pre-existing basement-involved fault. The fault has high activity intensity, cut deep down to the basement and connects the Yuehai Formation. Therefore, the deep-seated fault adjacent to the gas reservoir is the key to the migration of CO2 gas to the shallow reservoir. The process of oil and gas accumulation in Oilfield A has the characteristics of "two types of crude oil, two stages of crude oil filling and one stage of long-distance CO2 flooding". CO2 flooding enhances the dynamics of long-distance secondary migration of oil and gas. In conclusion, the discovery of inorganic CO2 gas reservoir expands the exploration horizon and boundary of the Enping Sag and even the northern fault terrace zone of the Pearl River Mouth Basin.