Relationship Between Pre-Tertiary Unconformity And Hydrocarbon Migration-Accumulation In Jizhong Depression

Jizhong depression which is located in the northwest of the Offshore Bohai Bay basin is a Mesozoic and Cenozoic basin which went through several periods of superimposition and reformation. Since the Paleozoic, it has undergone multi-periods tectonic movement such as Caledonian, Hercynian, Indo-China and the Himalayan, and formed several unconformities and different types of buried hill. Especially in the late Yanshan, the North China platform was uplifted as a whole, and suffered long-period weathering and erosion, forming a regional unconformity. This paper is titled "Relationship between Pre-Tertiary unconformity and hydrocarbon migration-accumulation in Jizhong depression". Unconformity can be divided into five types in Jizhong depression: truncation unconformity, fault-fold unconformity, parallel unconformity, truncation-onlap unconformity and fault-onlap unconformity by massive two-dimensional and three-dimasional seismic interpretations . Different types of unconformity have different distribution law. Logging, well logging, core, field recording and other datas show that unconformity vertical structures are obvious in Jizhong depression. Because of the differences of time, manners, objects, palaeoclimate and palaeoenvironment, unconformity vertical structure can exist different combination forms. Study area has mainly developed 11 kinds of type, and the role of controling the oil is also different. Study of physical simulation experiment for reservoir-controlling of unconformity vertical structure shows that the unconformity structure of sand and gravel layer may be the migration pathway and accumulate oil. In gas injection experiment, only the higher part of unconformity structure can be the gas migration pathway and aggregation sites. Oil and gas is injected into the semi-weathered rock first, if semi-weathered rock is closed by the side, then basal conglomerate above the unconformity surface will be the migration pathway, and the oil and gas is accumulated in semi-weathered rock. Scanning electron microscope, core observation, field exploration and drilling statistics show that the semi-weathered rock solution hole in study area develop very well, its thickness is generally from a few hundred meters to several kilometers, which are good reservoir. Buried hill can be divided into two major categories, landform hill and tectonic hill, and subordinates 6 types in Jizhong depression;Oil and gas travel into the buried hill through fault and unconformity, and theirs migration pattern includes two categories: in-source and out-source. Generally oil and gas migration distance is less than 10km, because of the effects of the oil sources and other factors. There are mainly five types of buried-hill oil and gas reservoir relating to unconformity, which are single fault type, fault horst type, step-fault type, erosion and fault-fold type, and the first three types are developed than the others in Jizhong depression.