| Gaoyou sag in Subei basin has the largest amount of petroleum resources，which has great industrial values. In recent years of exploration, the number of newlyexplored structural oil and gas traps decreased while the number of foundedlithological traps increased gradually. The key point of exploration is varying fromstructural traps to subtle ones. Finding favorable sands by the methods ofsedimentology and understanding the characters and laws of hydrocarbonaccumulation in the sag became more and more important.Gaoyou sag in subei basin is a terrestrial rift-subsidence lacustrine basin formedfrom Late Cretaceous Period to Paleocene. Huangjue-majiazui district, with theconsistent tectonic evolution characteristics of the whole depression, lays on the westpart of Gaoyou sag, but has a much more complicated tectonic evolution processcaused by the two big controlling faults(Zhengwu&Hanliu fault zone) passedthrough this district. The research target is a small fault-complicated fault-blockoil-gas field, with properties of broken blocks, complicated sand and hydrocarbonaccumulation distribution. Exploring and developing process was plagued by thosefactors above.Even though Huangjue and Majiazui district were geographically nearby, theresearch work of the two districts were separated for decades, while the research onstructure, sedimentary, reservoirs, oil and gas accumulations are unrelated. But thispaper discussed the provenance, sedimentary facies, and genetic mechanism of sandbodies based on tectonic evolution by the integration geological materials of cores,well-logging data, seismic data, and tectonic evolution information and so on as awhole. The genesis of reservoir evolution is discussed after those researches. With thedistribution of sedimentaries, source rocks, hydrocarbon transport system,hydrocarbon accumulating process, and the distribution of petroleum were studied.Normal research on provenance was mainly based on mineral indexes, anddivided the provenance directions. In this paper, the methods of provenance researchis not only based on minerals but also combined with structural geology ways todescribe the details of provenance. With explained seismic profiles and planecombination forms of faults, fault troughs, fault accommodation zones and incisedvalley transportation systems were recognized. Fault-step slopes and steep slopes transportation systems were found as well. All those transportation systems had greatability on transporting debris, and also controlled the direction of transportationdirections and the thickness of sediments. According to stable heavy minerals andmineral compositions, three directions were described, which came from south, northand west. With the structural researches, the details of provenance were depicted.Architectural patterns of structure determined the paths of provenance.The sedimentary characters were studied based on the process of“points-lines-surfaces-bodies”. With the idea of keep the importance and not let go thesuspects, we believed that the sedimentary facies has the attributes of controlling byrise and fall of water surface. In E2d1period, lake-level rose, it developed near-shoresubaqueous alluvial fan, lacustrine, and slump-turbidity fans facies. Up to E2d2period,lake-level fell; it developed fan deltas, lacustrine, and slump-turbidity fan facies.Based on the relationship of hydrodynamic conditions and tectonics, the causes andmechanism of sands were researched. Sands under the influence of tractive currentand gravity flows developed in the area of Dainan formation. Compared with theresults of previous studies, some suspected sedimentary structures were studied.Paleo-earthquakes and paleo-storms were identified. Those phenomenons andessences explained the sands found in deep-lake muds. It is considered that gravityimbalance and external excitation caused the slump-turbidity fans forming. A newdirection for oil and gas exploration was provided.Despite the limitations of formations and layers, the reservoirs were studied. Bythe characters and digenesis process, we found the physical property of reservoirs isnot determined by formations but mainly by the burial depth they lay. Differentchemical action caused by temperature field and formation water chemical acts causedby different burial depth of different formation in Huangjue and Majiazui district; thedigenesis of reservoirs were different. In vertical dimension, compaction andcementation which happened in early diagenesis stage A to B decreased the porosityand permeability until middle diagenesis stage A, and then secondary dissolutionbegan to enlarge the pores and improve permeability. With the different buried depth,it shows the phenomenon that diagenesis stage in the east is higher than the west, thenorth and south are higher than the middle. Sedimentary facies controlled the physicalproperties on ichnography. Reservoirs of those subfacies, such as Fan Delta Front,Near-shore Subaqueous Alluvial Fan Middle, have better physical properties than onthe other subfacies. All those above cases were concluded the feature of “sedimentary controlled reservoirs” and “buried depth controlled diagenesis”. Using physicalproperties “styleⅡ and Ⅲ” coupled with diagenesis stages “EA, EB, MA, MB”,“styleⅡ” are the favorable reservoirs and this kind of reservoirs appeared at early tomiddle stages. Reservoirs and seals well matched in research area, formed severalgood reservoir-seal combinations.Based on the previous data investigation and comparison of source rocks, E1f4formation is considered to contain the best source rocks in Gaoyou sag and is the besthydrocarbon provider for Dainan formation with the organic matter of typeⅠand typeⅡ. After oil-and-source-rock-correlation research, source rocks in Shaobo Sub-sagwas confirmed as the main source rocks for Dainan formation in research area, whichdeveloped nearest at research area. They were considered of the best type of organics,high in organic matter, and evolution, after studied the index of “Ro”,“chloroformbitumen “A””,“HC”,“TOC”,“S1+S2”, et.al.The research of hydrocarbon accumulation is mainly about oil and gastransporting system and oil-gas preservation. Transporting system, especially the faulttransporting system, played a very important role in petroleum transporting of a faultbasin. Oil and gas transporting system made of skeleton sand bodies, faults,unconformities and their combination forms were studied and evaluated. Faults ofdifferent level have different effect ions. Level1and2faults connected source rocksand reservoirs when they were activated while static period they sealed the oil and gas.Level2and3faults have certain capability on controlling oil and gas accumulations.They adjusted and rebalanced oil and gas distributions when active or sealed whenmotionless. Space-time distributions of favorable facies belts controlled thehydrocarbons. It can concluded that faults of high level cut through source rocks andtransferred hydrocarbons, small and low-level faults seal while sand-bodies restrainedthe plane distribution and rebalanced existed hydrocarbon reservoirs.Slump-turbidity fans developed in deep lake zone produced the play ofsource-rocks, reservoirs, and seals through the research, those sandstones originatedfrom fan delta front or near-shore subaqueous alluvial fan middle, under the externalforce of earthquakes or hurricanes or internal force of gravity, will one day becomethe new hope and focal point for further exploration on lithological reservoirs. |
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