Research On Gas Enrichment Regularity And Accumulation Mechanism Of The Upper Paleozoic Gas Reservoir In Hangjinqi Area

Hangjinqi block is located in northern Ordos Basin,.The research area is separated into north part that belongs to Yimeng uplift and south part that belongs to Yishan slope by Boerjianghaizi fault zone. As the abundance resources and low exploration degree in Hangjinqi block, it becomes an important gas exploration area in Ordos basin and in recent years makes a breakthrough in exploration. In the process of exploration, it is found that every well produces water and the gas-bearing formations are different between the south and north. Because the unknown about the controlling factors of gas-water, gas accumulation and its mechanism restricts the further exploration and development, this paper aims to analyze the gas accumulation characters and its controlling factors, the relationship between abnormal pressure and gas accumulation, and accumulation mechanism through previous research and related theories such as petroleum geology and petroleum accumulation dynamics.The results are organized as follows:(1) Through the analysis of traps in every formation, it is suggested that devided two part by the Boerjianghaizi fault, in the north there are many types of traps such as lithologic trap which is the most common trap, lithologic-fault trap, anticline trap and unconformity trap,and in the south lithologic trap that formed by lithologic pinchout is very common except that structural-lithologic trap occurs in H1-3, and nosing structure and fault are not common.(2) The under compaction was development in whole research area. Vertically,the under compaction begins in Shiqianfeng formation and reaches maxium in upper Shihezi formation, then decreases in lower Shihezi formation, and finally disappears in Taiyuan formation. From southeast to northeast the start layer of high pressure formation had lower tendency( the start layer from the top Shiqianfeng formation to the mid-lower Shiqianfeng formation), the depth of disappearance layer of high pressure had decreasing trend( the disappearance layer from the Taiyuan formation to the top of Shanxi formation). Horizontally, from the southwest to the northeast uncompaction has gradually weakened, and the Boerjianghaizi fault zone was thedistribution zone of weakened uncompaction.(3) The development of abnormal pressure is influenced by sedimentation,structure and fault. Sedimentation( ratio between sandstone and stratum thickness)controlls the degree and plane distribution of under compaction. On a macro level, the structure uplift can release pressure, especially in the northeast of research area in which the pressure releasing is the most obvious. The pressure releasing from fault mainly reflect in the upper wall of Boerjianghazi fault and its influence range stretches to 200-300 m.(4) Based on the conclusion of gas and water characteristics, some methods such as resistivity difference, cross-plot and multivariate discrimination are used to effectively identify the gas and water and the standard of gas-water identification that is appropriate for research area is established, with good indentifacation effects. The formation water is divided into interlayer channelingwater, condensate water, lenses water, channel river bottom retained water, the edge and bottom water by water-producing characters and parameters of formation water chemistry and then the judging and identification criteria and distribution patterns of different types of formation water has been established.(5) In different areas and formations,the gas-water distributions have different characters. In the south, the upper formations produce water and the gas is produced in lower formations. In the north,the situation is opposite. The gas in the north is mainly distributed in high positions such as tectonic ridge and high point of anticline,but in the south gas is distributed in locations that have occlusion from lateral and updip direction. The gas is not productive in fault zone and the trap is relatively small.The gas and water are associated, and the gas-water interface is not unified.(6) There are obvious differences in gas transporting system,migration dynamics,trap types and controlling factors in the south and north. Through the analysis of gas accumulation of the south and north, it is found that in the north He2 and He3 are the main accumulation formations, and the effective traps in tectonic ridge and structural highs are the favorable accumulation locations, and the areas of low ratio between sandstone and stratum thickness and high overpressure value are the guarantee for gas accumulation. In the south He1 is the main gas-bearing formation, and the areas with low ratio between sandstone and stratum thickness in the edge of channel and locations that have occlusion from lateral or updip direction are gas enrichment areas.The areas in which underlying sand overlay in He2 and He3 and micro-cracks develop are also gas enrichment areas. In fault zone, the gas-producing is poor andgas column is low, and gas accumulates in small areas with better closure conditions.(7) As in the south and north of research area,there exists differences in trap availability, migration dynamics, transporting system and sealing conditions, gas accumulation has different characters between the south and north. In the north, most gas is from the south and only a small amount is from the north gas source.The transporting system is sandstone-fault-surface of unconformity and the gas is transported from south sandstone, fault and surface of unconformity to He1 sandstone and then through the sandstone-fractures transporting system the gas goes into the overlying strata so that gas can be identified in Shiqianfeng and Liujiagou formations.In the fault zone, gas is originated from the south and the transporting system is sandstone-fault in which gas migrates from sandstone and fault to sandstones that located in place nearby the fault. The gas accumulates in structural traps and structural-lithologic traps and the vertical sealing condition is not good enough to seal off the gas column. In the south,gas is originated from underlying Shanxi,Taiyuan formation and the main transporting system is sandstone,and sanstone-fault system only exists in the southwest corner. The migration dynamics is mainly from source-reservoir differential pressure and the buoyancy takes second place. With the good vertical preservation condition, the upward dissipation of gas is not obvious.(8) Based on the differences of gas accumulation, controlling factors and accumulation mechanism in different areas and formations, the differential accumulation pattern of the south and north is established. In view of the trap types,accumulation law and controlling factors, favorable exploration areas in different formation is predicted.