The Study Of Beach-Bar Sand Reservoir-Forming Rule In The 3rd Member Of The Funing Formation,Zhangjiaduo Oilfield,Subei Basin

Abstract: The Paleogene Funing Formation in Qutang sag Subei basin produced highly developed source rock, well reservoir-cap combination and powerful hydrocarbon migration. Some of the high-productivity wells in upper part of steep slope zone in Zhangjiaduo area show a good oil-gas exploration potential, but deep concave part and southern fault-break zones are located outside of fault anticline traps; thus knowing the capacity of hydrocarbon accumulation in this area is the key issue to the further exploration.Based on the research of lithofacies, logging facies, seismic facies and paleontological data, this study consider the 3rd member of the Funing formation is mainly developing shallow - semi-deep lake deposit. Thereinto, upper and lower sandstone member are shallow lake deposit and mid sandstone member is semi-deep lake deposit. Also, beach-bar sand is the main sandbody type. The characteristics of sandbody in the 3rd member of Funing formation are vertical multi-layer superimposed and horizontal beach sand with bar sand mixed-layered. Because during the sedimentary period, the deep sag area in the 3rd member of Funing formation is not deep water area, the whole sag had partly developed beach-bar sand bodies.By study on cast thin section, scanning electron microscope(SEM), X-ray diffraction, burial history analyses, the result shows that even the buried depth in the 3rd member of Funing formation is really large, abnormal high pressure caused by under-compaction inhibited reservoir compaction leads to weak compaction strength and developed reservoir intergranular pore. As the result of the high clay content and the earlier closed diagenetic system, fine-grain reservoir caused by beach-bar sand inhibited quartz secondary overgrowth and ferrocalcite cementing, reservoir is mainly composed of clay cement and compaction is the key control factor of reservoir property. However, the buried depth in deep sag area is large; the undercompaction is more developed. The abnormal pressure caused by undercompaction could counteract part of the overburden pressure caused by the increased buried depth; moreover, with weak cementation there are still high-quality reservoirs in the deep sag area. The study of abnormal high pressure, sedimentary facies and diagenesis shows that beach-bar sand with high pressure coefficient in deep sag area develops high quality reservoirs. The bar sand sediments have the characteristics of powerful hydrodynamic, coarse grained, low clay content, obvious post-diagenetic dissolution and dissolution content is easily taken out of the sand body by water, the physical properties index are higher than beach sand, especially the permeability.Through the establishment of mudstone compaction curve, the abnormal pressure could be found in Funing formation. Due to the abnonnal pressure member has characteristics of high interval transit time and low resistance, the cause is synthetically judgmented to undercompaction by chart correction method. Based on the establishment of the normal compaction curve, this study employs the method of Eaton to calculate the overpressure and pressure coefficient in abnonnal pressure member. By studying well constraint and using the seismic data to retrieval mudstone speed and calculating the top of the structural section map in the 2rd member of Funing formation, compiled the horizontal overpressure distribution map of source rock in the 2rd member of Funing formation. Using the relationship between reservoir porosity and Pc50, calculated the reservoir equivalent charging pressure; moreover, studying the hydrocarbon-filling degree of its each single layer by reservoir index.Based on the overpressure, fault system and reservoir heterogeneity, this study proposed that different structure positions and layers produced different reservoir-forming rules. The lower sandstone member has the characteristic of continuous accumulation; because of the overpressure, the hydrocarbon in deep sag area migrates like a piston, continuously accumulating from low penneability beach sand to higher position. Also, due to the upper sandstone member controlled by fault, the hydrocarbon accumulated in higher position firstly, then with the increase of hydrocarbon-filling degree, the hydrocarbon accumulated in lower position.Based on the research of this paper, innovative achievements as follow:(1) In deep sag area the 3rd member of Funing formation develops sandbody caused by beach-bar;The sandstone in the 3rd member of Funing fonnation develops four lithofacies and four logging facies, combining datas of seismic facies and paleontology, this study identified that the type of sandstone in the 3rd member of Funing formation is beach-bar sand caused by shallow lake. Using the method of geophysics, describe the distribution of sand body in layer by layer. The beach-bar sand being developed in the deep area of sag, shows that the sag is not formed when the the 3rd member of Funing formation had deposited. This research achievement confirmed that distribution of sand bodies could exist in the deep sag.(2) The beach-bar sand in abnonnal high pressure area is the development area of high quality reservoir;The characteristics of sandbody are fine-grain size, high compositional maturity and high structural maturity; because of the beach-bar sand surrounded by lacustrine mudstone, the disequilibrium compaction could cause the abnormal high pressure. Moreover, the abnormal high pressure weakened the effective stress which affects on skeletal grains and the intergranular pore could be preserved. Using the method of geophysics, predicting abnormal high pressure area, the result shows that there is abnormal high pressure in deep sag area. Meanwhile, because of the high clay content, the pore specific surface area of the fine grain beach-bar sandstone is high and it could inhibit the deuterogene siliceous cementation and carbonate cementation. The low clay content of bar sand causes siliceous cementation; however, its cementation degree is low and corrosion of low clay content bar sand is well developed, lead to secondary pore developed. Thus, bar sand reservoir property is better than beach sand. The beach-bar sand grained size is coarser, its permeability is better. This research achievement confirmed that high quality reservoir could exist in the deep sag.(3) Established continuous accumulation model of lower sandstone member and discontinuous accumulation model of upper sandstone member in the 3rd member of Funing formation in studying area;Due to the lower sandstone member next to the source rock, oil and gas generated by hydrocarbon kitchen migrate like a piston, continuously break through the bottleneck of beach sand to higher position, the accumulation process is the process of energy gathering formation, the energy break through to migration, energy gathering again, and energy break through to migration again, this accumulation process is continuous accumulation. Thus lower position enriched hydrocarbon and higher position enriched water. Because of well Qu-1 in upper sandstone member with antithetic fault far away from hydrocarbon kitchen, oil and gas had mainly accumulated from lower sandstone member. The oil and gas in lower sandstone member passed through fault and migrated to higher position, accumulation in the upper sandstone member, with the increase of migrated hydrocarbon, oil and gas begined to migrate to lower position; this accumulate process is discontinuous accumulation. Thus the upper sandstone member has the characteristic of lower position enriched water and higher position enriched hydrocarbon. This research achievement confirmed that accumulation reservoir could exist in the deep sag.