白云凹陷断裂作为天然气运移通道的地质-地球化学证据

通过对白云凹陷区域地质特征分析,并依据天然气的气体成分和甲烷与乙烷的碳同位素关系,对天然气的母源进行分析。结果表明,本区天然气的母质类型主要为II—III有机质生源。对坡折带断层活动的时限进行分析,发现白云凹陷的断裂活动可分为I—IV4个区,每个区的断裂活动均有差异。从I区到IV区,断裂活动依次减弱。番禺低隆起反向断层东西向和南北向活动性存在差异:第三排反向断层活动性强于第二排、第四排又强于第三排。反向断层的主活动期对油气的垂向运移具有重要控制作用,而结束期对油气的封堵圈闭有重要意义。依据录井天然气的同位素特征和流体包裹体的分析证实了断裂是本区天然气垂向运移的主要通道,断层的活动与罐装岩屑录井气地球化学特征完全一致。PY34-1-1断层活动微弱,热成因气没有运移到浅层,说明该区天然气的运移主要发生在深层T5层。LH19-1-1在T4—T5、T2层均发生了天然气的运移,甚至在1000m的浅层,也发现了甲烷碳同位素很重(-30‰)的热成因甲烷。这说明在LH19-1-1区块,热成因气已经运移到浅层。与此同时,流体包裹体均一化温度的直方图与罐装岩屑录井气中的非常吻合,断层活动越强,包裹体均一化温度的峰值越多,这同样表明天然气的垂向运移和分布主要受断裂的活动控制。

The geological and geochemical evidence on the identification of natural gas migration through fault system, Baiyun Sag, Pearl River Mouth Basin, China

The origin of natural gas has been analyzed based on the regional geological characteristics of Baiyun sag and carbon isotopic composition of methane and ethane. The results show that the natural gas is mainly derived from type II-III organic matter. The analysis of fault activity near slope shows that the four zones can be classified based on the fault activity intent with geological time. The fault activity intent decreases from zone I to IV. In addition, the fault activity varies between east and west, north and south. The fourth line fault activity is more intense than the third line fault. In turn, the third line fault activity is more intense than the second line fault. The fault active period controls the gas migration, and close period can be as a seal for a gas trap. The geochemical characteristics of the adsorbed gas in sample log verified that fault do play an important role for gas migration. The fault activity coincided with the isotopic composition of the adsorbed gas. For example, the shallow strata in well PY34-1-1 are biogenic gas, and thermal gas is mainly in the deep part of T5 strata with indistinct fault activity in this region. In contrast, the shallow strata in well LH19-1-1 are mainly thermal gas because of intense fault activity. The carbon isotopic composition of methane of adsorbed gas is ca.-30‰, and this shows the thermal gas migration in the shallow strata through fault. In the meantime, the homogenization temperature of fluid inclusion in different wells also coincided with the adsorbed gas characteristics and fault activity. As the fault activity is intense, more peaks of homogenization temperature were recorded as the fluid activity appeared. These evidences prove that the fault mainly controls the migration and distribution of gas vertically.