| Xihu depression whose area is about5.9×10km2is in the northeast of the East China Sea Basin, and is the focus of oil and gas exploration. So far, more than50exploration wells has been completed. Thirteen oil and gas fields and thirteen oil and gas structures has been found. There are good oil and gas prospects. However, oil and gas fields explored have a small scale in Xihu depression, which has big oil and gas bearing structure, low filling degree and poor reservoir physical properties, which restrain oil and gas production growth. So, the study of accumulation rule and control factors is an important problem.Taking the central inversion tectonic belt in Xihu depression as an example, this paper apply some theories such as petroleum geology, fluid inclusions geology, petroleum geochemistry, accumulation dynamics and tectonics and a large number of drilling data, geochemical testing and fluid inclusion system analysis technology to research accumulation conditions, characteristics and control factors from source rock, reservoir rock, seal rock and tap and matching relationship. Main results for this study are as follows:1) Coal-measure source rock of Pinghu Formation is the primary resource rocks falling into coal, carbon mudstone and dark shale. Coal rock have highest abundance of organic matter, followed by carbon mudstone, lowest is dark shale. Reservoir of Huagang Formation, as normal reservoir is relatively well, while Pinghu Formation, as low-permeability tight sandstone is relatively poor. The high shaly content (>40%), big mono-layer thickness, steady distribution and undeveloped cracks leads the mudstones in the upper part of Huagang Formation the regional cap rocks. Capillary pressure is the main trap mechanism of cap rocks. Influenced by tectonic movement and sedimentary environment in different periods, various types of entrapment are developed. Structural trap is the main style, followed by carbon mudstone, while lithologic trap is distributed regionally.2) Natural gas is mostly coal-type gas identified by stable isotope of carbon, and gas of TWT Well1is deep source gas. Computed by geochemical parameters, The maturity of natural gas is in mature-high mature stage, and the oil is in mature stage. oil-source correlation illustrates that petroleum mainly comes from coal series resource rock of Pinghu Formation.3) Mainly distributed in central and southern structural belt, gas and oil is found in large and medium-sized extrusion anticline structure, in Huagang and Pinghu Formation, and under a huge cap rock. Gas and oil with generally longitudinal distribution develops conventional gas reservoir in the upper part, and low porosity low permeability gas reservoirs in the deep part.4) In the late faulted period, Xihu Sag experiences three stage of structural inversion. The first stage occurs in late Eocene(about35Ma) caused by Yuquan Movement. The secod stage occurs in late Oligocene (about25Ma) caused by Huagang Movement. The third stage caused by Longjing Movement is the most obvious one with extensive affects. Drape and thrust fault is mainly distributed in central structural belt.5) Basin modeling demonstrated that hydrocarbon source rocks of pinghu group of the central inversion tectonic belt had a high hydrocarbon generation rate in Miocene and nowadays, and a high hydrocarbon expulsion rate in middle Miocene. Fluid inclusion system analysis technology demonstrated that there are three stages of petroleum filling:the early low-mature yellow fluorescence oil charging in21Ma～16.2Ma; the middle mature blue-green fluorescence oil charging in10.4Ma～6.1Ma; the late high maturity blue fluorescence oil and gas charging in2.2Ma～0Ma. The later two stages are the most important accumulation period of the study area.6) Controlling factors of petroleum entrapment in Huagang Formation are as below:(1) cap rocks control the scale of gas pool;(2) faults control the accumulation of oil and gas;(3) anticline structure control the distribution of oil and gas;(4) matching relationship of accumulation elements control pool forming; Low porosity low permeability oil and gas resources develop in Pinghu Formation, and relative structural high is not favorable area of petroleum accumulation. The factors controlling the cracked gas accumulation are resource controlling and phase controlling. Petroleum is stored in area of good physical characteristics and huge hydrocarbon expulsion. Exploration target is mainly " sweet point ". As a result, there are two primary accumulation model in study area. One is represented by Huagang Formation with characteristics of " Down generated Up stored", multi-stage petroleum pool-forming and controlled by large anticline and faulted anticline. The other is represented by Pinghu Formation with characteristics of " self-generation, self-reservoir", multi-stage petroleum pool-forming and controlled by reservoir physical property and expulsion intensity of hydrocarbon. |
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