The Mechanism And Main Controlling Factors Of Hydrocarbon Accumulation In Liaoxi Depression

The Liaoxi depression is located in the northwest of Bohaiwan basin, it is the offshore extension of onshore western Liaohe depression in Xialiaohe depression, Liaoxi depression and western Liaohe depression are two sub-depression belongs to Xialiaohe depression, the two depression have the same tectonic setting and sedimentary sequence and similar structure, according to ghe successful exploration practice of onshore western Liaohe depression, the difference and commonness analogy between the hydrocarbon accumulation conditions of these depressions will provide guiding for understanding the accumulation regularity of Liaoxi depression. Since1986,16structures have been drilled, with three oil fields, seven oil and gas construction found, the enrichment difference among the north sag, center sag and south sag is big, the analysis of petroleum accumulation mechanism and main controlling factors of hydrocarbon accumulation in Liaoxi depression has important theoretical guiding significance in the exploration of study area and other similar areas.Starting from the basic geological conditions and tectonic background of the Liaoxi depression, based on the comprehensive utilization of the data of geology, geophysics and geochemistry, under the guidence of multi-disciplinary theories, this paper studied firstly the elements of hydrocarbon accumulation such as source rock, reservoir, seal rock, source-reservoir-seal assemblage,trap and then make a comparative analogy of geological conditions in Western Liaohe depression sag. After that, the accumulation progress was established, oil and gas enrichment regularity were analyzed, and through anatomy of typical reservoirs, division, the major patterns of oil and gas accumulation were established and the control factors of petroleum accumulation are discussed. The results are as followed:The elements of hydrocarbon accumulationBased on geochemical data, the organic matter abundance, types and maturity of source rocks in Liaoxi depression were evaluated, the results show that there are three sets of source rocks in Liaoxi depression namely Ed3, Es1+2and ES3, among them the ES3source rock have large thickness, high abundance of organic matter type, Ⅰ-Ⅱ kerogen type and high maturity is the largest contributor of accumulation in the study area; Es1+2source rocks with high abundance of organic matter and Ⅱ kerogen type, high maturity, thin thickness, is a set of important hydrocarbon source rocks in the study area; Ed3source rock with general abundance of organic matter,Ⅱ-Ⅲ type kerogen, general maturity, large thickness, is a minor contributor to the petroleum accumulation.There exist delta sand body, sand bodies of braided river delta, fan delta sand body, nearshore subaqueous fan sand body and turbidite fan sand body deposit in Liaoxi depression; two kinds of reservoir spaces as primary pore and secondary pore, the secondary pore developed well, from top to bottom there are three major zones for the development of secondary porosity belts, mainly affected by the late diagenesis and dissolution, etc; the physical condition of reservoir in Liaoxi depression is superior with average of25.6%, the average permeability of479.3mD, most reservoir porosity more than20%, most permeability more than100mD, belong to type I reservoir with high porosity and permeability.There are three sets of regional cap rock deposit in Liaoxi depression, namely Ed1-Ed2U, Ed2L-Ed3and ES3, among which, the Ed2L-Ed3regional cap rocks are the most important and widely distributed in the whole study area with strongest capping ability. Reservoir-seal associations in Liaoxi depression can be roughly divided into three categories:self sourced reservoirs, source rocks in the upper part and reservoirs in the lower part and source rocks in the lower part and reservoirs in the upper part.The trap type developed in of the Liaoxi depression is unitary, mainly belong to structural trap related to the faults, pure lithologic trap and stratigraphic trap are rare, structural trap can be divided into faulted anticline, rolling anticline trap structure, draping anticline, fault nose and fault block5categories, the size of the traps is small, with average closed area of about16.3km2and average close range of178.85m.The conducting system in Liaoxi depression is mainly composed of fault, sand body and unconformable surface, which can be divided into three kinds of faults namely the single fault element type, fault-sand body composite and composite unconformity, due to well developed oil source faults, trap mostly distribute dependent on oil source faults. Therefore, transportation systems in Liaoxi depression is given priority to with single fault element type, fault, sand body compound are developed, unconformity-fault rarely found.Hydrocarbon accumulation mechanism and enrichment regularityBasin simulation technology was used to study the buriing processes and evolution, hydrodynamic field and evolution, development and evolution and hydrocarbon source rocks of oil and gas filling history in Liaoxi depression. Results show that affected by tectonic evolution, buried formation has experienced different sedimentary changes and characteristics respectively corresponding to the tectonic evolution of four phases:during the Kongdian-Shasi period, the earth’s crust begin to arch, sedimentation rate decrease, settlement become smaller, the whole depression become a segmented lake basin; during the ES3period, the rupturing is strongest, sedimentation rate increased, the subsidence quantity also increased; during the ES1+2period, the rupturing weaker, sedimentation rate become smaller and subsidence is minimal; during the Dongying group sedimentary period, the recurrence of lacunae, formation deposition rate increases again, thickness increased; After Neogene, formation and sedimentary being stable buried, entered the stage of depression..Nowadays, measured temperature data revealed that the geothermal gradient in Liaoxi depression mainly distributed in2.2℃/100m to3.7℃/100m, average geothermal gradient at2.95℃/100m, slightly lower than the average of Longdongwan (3.0℃/100m), basin numerical simulation results revealed that the palaeogeothermal and palaeo-heat flow in Liaoxi depression has experienced relatively similar change process, which is from high to low first, and then continue to rise during the period of geologic history. The paleogeothermal gradient and palaeo-heat flow is higher during the chasmic strong stage, when the chasmic function is weak, they reduced correspondingly.Measured pressure data revealed compaction, mudstone overpressure phenomenon in the study area, according to mudstone acoustic time difference method, there are two sets of mudstone undercompaction and overpressure system in the study area, namely ES3formation and Lower Ed2-Ed3formation, and the overpressure intensity in the deep overpressure system is larger than shallow overpressure system, but the shallow overpressure system distributes wider.Basin numerical simulation analysis show that there are three periods of pressure-pressure release evolution cycle in the study area, and strong tectonic movement and high sedimentation rate, there exist strong undercompaction overpressure during the Es3sedimentary period,; druing the ES2sedimentary period, the tectonic movement is abate, pore fluid seepage, partial pressure release, overpressure intensity decreases; during the Ed3sedimentary period, tectonic movement intensity increase again, compaction increase, pressure accumulated; during the Ed2sedimentary period, the pressure should be reduced, but part of the hydrocarbon source rock began to expulsion hydrocarbon, lead to increase pressure, until the end of the east section of sedimentary strata uplift, due to the tectonic uplift and denudation, the pressure release; during the Minghuazhen formation, the source rock entered the hydrocarbon generation peak, pressure increase correspondingly; during the Quaternary Pingyuan formation sedimentary period, the hydrocarbon generation abate, pressure decrease again. These are the three phase of the pressure evolution cycle.Sand basin numerical simulation results show that the ES3source rocks entered hydrocarbon generation threshold during the Es1+2formation sedimentary period, began to expulse hydrocarbon during the Dongying formation sedimentary period, reach the peak of hydrocarbon generation during the Guantao formation sedimentary period, reach the peak of hydrocarbon expulsion during the Minghuazhen formation sedimentary period; part of hydrocarbon source rocks from Guantao formation entered hydrocarbon generation threshold during the Minghuazhen formation sedimentary period and a small amount of hydrocarbon were expulsed, reached the hydrocarbon generation peak during the Minghuazhen formation sedimentary period, reach the hydrocarbon expulsion peak during the Pingyuan formation sedimentary period; very small amounts of Ed3source rocks entered hydrocarbon generation threshold and expulsed hydrocarbon during the Guantao formation sedimentary period, however the expulsion amount is rare, with minor contribution to the accumulation.Combining previous By combine the fluid inclusion test and analysis with the use of traditional generation and expulsion history of hydrocarbon, the results show that the mass migration and filling time of oil and gas in Liaoxi depression is roughly at24.6Ma, that is to say during the Guantao formation sedimentary period.The physical properties of crude oil in Liaoxi depression varies with each other, most deep buried oils are light oil and intermediate oil, and shallow buried oils are mainly heavy oil; according to the hydrocarbon source rocks sterane maturity parameters, the types of crude oil in Liaoxi depression are divided, including immature oil, low mature oil and mature oil, their maximum depths are2500m,2780m and2900m respectively. Oil-source correlation results show that the Liaoxi depression mainly have a hollow accumulation rule, does not exist migration phenomenon between pools; the enrichment regularity in the study area show the oil and gas resources are abundant in north-south, with center area deficient on plane, deep layers abundant and shallow layers poverty vertically, high position abundant and low poverty laterally. Controlling factors and models of hydrocarbon accumulationSeveral typical reservoirs were selected in different sub-sag, and different structural belt among the Liaoxi depression, then the accumulation conditions, the accumulation mechanism and main control factors of these reservoirs were analyzed, on this basis, according to the hydrocarbon sub-sag, oil and gas source, oil and gas reservoirs, conducting system and the degree of enrichment, the hydrocarbon accumulation model of Liaoxi depression can be divided into three categories and11sub-types, then the main control factors and distribution characteristics of oil and gas reservoirs were analyzes.The hydrocarbon accumulation pattern of Liaoxi depression can be divided into three categories namely deep mixed source old reservoir type, one-way shallow old reservoir type and unidirectional composite new reservoir, in particular, the main controlling factors of deep mixed source old type reservoir are reservoir quality and conducting channel, main controlling factors for one-way shallow old reservoir type are hydrocarbon source rock condition, migration dynamics and assemblages; control factors for unidirectional composite new reservoir are caprock and faulting.The distribution model of these three types of the reservoir is obvious:deep oil stored reservoirs are mainly distributed in the north sag of Liaoxi depression; shallow old reservoir distributes widely in the north sag, center and south sag; new storage reservoir distributes only in the south sag. Various kinds and patterns of hydrocarbon reservoirs distribution also follow a certain law:rich reservoirs are mainly distributed in the concave structure zone and the steep slope zone; mix sourced gas reservoirs are mainly distributed in the north sag and the south sag.The different oil and gas enrichment mechanism in Liaoxi depression is controlled by comprehensive factors such as hydrocarbon source rocks, fracture, reservoir temperature pressure system, the control function of the various elements is shown as:high-quality hydrocarbon source rocks development deicide the distribution area of oil and gas, the hydrocarbon generation center control the enrichment area of oil and gas, the hydrocarbon source rock thermal evolution affect the difference of oil and gas distribution, intence fracture activity provides development condition for the high quality hydrocarbon source rock, tectonic evolution cycle led to the formation of multiple sets of reservoir-seal associations, and fracture distribution control the trap development, late activation of fractures improved oil and gas migration channel, double entry temperature pressure system manufacturing " fluid compartment", oil and gas mainly gathered on the edge of the overpressure system, reservoir quality affect the overall oil-gas accumulation scale.