海拉尔盆地贝中次凹——残留型叠合小断陷盆地油气勘探的成功案例

Beizhong Sub-Depression in the Hailaer Basin:A successful case for oil exploration in small residual superimposed rift basins

贝中次凹是海拉尔盆地贝尔凹陷内的一个小型断陷,面积为260km2,主体勘探面积仅110km2,其下部为富含火山物质的残留断陷,上部为遭受严重剥蚀的断坳和坳陷。2005年以来,以残留型叠合断陷盆地勘探理念为指导,通过实施三维地震和不断深化地质研究,在贝中次凹探明了5 000×104t级规模储量,成为小凹陷获得大发现的成功案例。贝中次凹勘探经验揭示,"残留"和"叠合"的特点可能是小凹陷富含油气的关键因素。"残留"意味着原始沉积时断陷规模有可能很大,与未遭受过剥蚀的小凹陷相比,残留凹陷通常有效烃源岩比例要大,油气资源丰度相对要高,而且,富含火山物质的残留断陷期储层易溶蚀形成次生孔隙;"叠合"意味着其上的断坳期地层可作为良好的区域盖层,多期不整合面和断裂有利于油气运移聚集。东北亚地区中生代断陷盆地广泛分布,其中发育众多的残留型叠合小断陷,贝中次凹勘探的成功预示着这些小断陷可能具有类似的油气成藏优势,是下一步实现勘探突破的方向。

The Beizhong Sub-Depression is a small rift in the Beier Depression of the Hailaer Basin.This sub-depression covers an area of 260 km^2,of which the main exploration area accounts for only 110 km^2.Vertically,strata of this sub-depression can be divided into two parts,the lower one is composed of a residual rift rich in volcanic materials and the upper one consists of severely eroded thick fault-sag and sag deposits.Under the direction of a residual superimposed rift-basin concept,over 5 000×10^4 tons of the oil reserves have been proved in this sub-depression by means of 3D seismic survey and persistent geological studies since 2005,which makes the Beizhong Sub-Depression a successful case of a great discovery in a small rift.The exploration experience in the Beizhong Sub-Depression reveals that ＂residual＂ and ＂superimposed＂ features may be the key factors that result in oil-gas richness in such a small rift.The ＂residual＂ means that the original scale of the rift used to be very large during deposition and source rocks were likely to develop well then and survive more later in a residual rift,which makes the ratio of the effective source rock to gross higher for residual rift compared with small uneroded rifts.Furthermore,a residual rift reservoir rich in volcanic materials is easy to get better secondary porosity through corrosion.The ＂superimposed＂ means that overlying fault-sag strata could form good regional seals,and multi-phase unconformities and faults might provide favorable migration pathways and traps for oil accumulations.Numerous small residual superimposed sub-depressions are developed in Mesozoic rift basins that are commonly distributed in Northeast China and Northeast Asia,they may have oil accumulation preponderance similar to the Beizhong Sub-Depression and are,therefore,likely to be new targets for further exploration.