Depositional Systems And Exploration Of Subtle Oil-gas Pools Of Paleogene In Jizhong Depression,Bohai Bay Basin

The oil and gas has been explored for 50 years in Jizhong Depression, Bohai Bay Basin. A large number of enriched and largeduty "Buried hill oil-gas pools" of proterozoic and palaeozoic, as well as structural oil-gas pools of Paleogene, have been found in the study areas. These achievements of exploration have made important contributions for the development of Chinese petroleum industry. However, along with higher explorational degree of "buried hill" and structural oil-gas pools, it is more and more difficult to find them and the explorational situation is facing a hard test. On the other hand, the residual hydrocarbon resources of Jizhong Depression is still large and most of them are likely to be accumulated in subtle traps with lower explorational degree. It can be forecasted that subtle oil-gas pools will become the most important and realistic explorational domain for finding a large scale of geological reserves.On the basis of reading many references of the predecessors and absorbing main research and exploration achievements obtained in recent two years of Huabei oil company, and using sufficiently large numbers of the data of wells, logs and seismic lines in Jizhong Depression, the depositional systems and characteristics of Paleogene in Jizhong Depression is studied first as a beginning of the paper. And then, the formational conditions, controlling factors, distributional characteristics and accumulation rules of subtle oil-gas pools are analyzed in detail. The types and formational models of subtle oil-gas pools are classified, and the technologies and methods of subtle trap identification are summarized. In the end, the paper points out the favorable directions for exploration of subtle oil-gas pools in the study areas.It indicates that seven types of depositional systems of Paleogene in Jizhong Depression, including drift bed or alluvion(diluvial), fluvial, fan delta, braid delta, delta, sublacustrine fan and lacustrine system, are developed. The evolution of lacustrine basins has passed through a integrated changing process which can be divided into the early stage isolated and separated, middle stage widened and connected and late stage shrank and died out. As a result of evolution of lacustrine basins and development of depositional systems, three sets of main hydrocarbon source beds and nine types of reservoir bodies divided by their origin are formed, and they provide the favorable physical basis for the formation of subtle oil-gas pools. At thesame time, the undercompaction process which occurs abroad in deeplying reservoir bodies expands the explorational space of subtle oil-gas pools to a deeper position. In the transverse and vertical directions, "five zones" distributed around lacustrine basins, including variable zone of lacustrine strandline, transitional zone of lithofacies, overlapping and erosive zone of strata, favorable lithofacies zone and diagenetic facies zone, are the advantageous zones to form subtle oil-gas pools. However, the unique depositional and structural settings of palaeogeomorphology and structural inversion have some stronger controlling processes for development of subtle oil-gas pools.According to the origin, geometry and seal elements of traps, the paper classifies the subtle oil-gas pools of Paleogene in Jizhong Depression into the following types: lithologic, stratigraphic and composite oil-gas pools. Furthermore, they can be subdivided into ten subtypes and seventeen kinds of subtle oil-gas pools. Their distributional characteristics are different in different kinds of sags such as Langgu, Raoyang and Shulu sag, and form accordingly many distribution or accumulation areas of different types of subtle oil-gas pools. On the other hand, because the geologic frameworks of the different zones and different positions of the same zone, including steep slope, trough region and gradual slope of half-graben sags, are complicated and variable, their formational models of subtle oil-gas pools in these areas are different. Moreover, the differences of conducting systems and accumulating degree between the different types of subtle oil-gas pools are also obvious.In connection with the technical difficulties of identification about subtle traps, the paper summarizes systematically five kinds of key technology and a flow chart, and sums up specially the methods to identify the thin interbed sandstone bodies, heavy and fan-shaped sandstone bodies and fissured reservoir of special lithology. Finally, the advantageous explorational zones and targets are selected and evaluated about the subtle oil-gas pools of paleogene in Jizhong Depression. The result of evaluation shows that it can be divided into nine zones as the first types, six zones as the second types and five zones as the third types for exploration of subtle oil-gas pools. At present, a large number of favorable targets have been found in these explorational zones, and indicate a better prospect for exploration of subtle oil-gas pools in the study areas.