| The Pearl River Mouth Basin is the largest Mesozoic-Cenozoic sedimentary basin in the northern South China Sea, and is also an important oil and gas resource base in offshore China. Among it, the Zhu I Depression has been confirmed to be the region with the best commercial exploration results. Previously discovered oilfields and oil-bearing structures are mainly distributed in the hydrocarbon-rich sags of Huizhou, Enping, Lufeng and (Panyu4Sag) and its surrounding areas, these oil and gas resources account for about62%of the total resources in the Zhu I Depression. Therefore, carrying out research on hydrocarbon accumulation regularity and the master control factors of the confirmed hydrocarbon-rich sags in the Zhu I Depression will provide some theoretical revelations and practical reference value for low proved oil and gas exploration areas.Starting from the basic geological conditions and tectonic background of the Zhu I Depression, based on the comprehensive utilization of the data and techniques of geology, geophysics and geochemistry and under the guidence of multi-disciplinary theories, this paper studied firstly the static elements of hydrocarbon accumulation such as source rock, reservoir, seal rock, source-reservoir-seal assemblage and trap, systematically. Using the combination research approachs of static description and dynamic simulation, macro and micro features, qualitative analysis and quantitative calculation and taking the techniques of organic geochemistry, fluid inclusions analysis and basin numerical simulation, this paper analyzed the accumulation dynamic conditions and process such as oil and gas source, migration dynamics, the conduit system, accumulation time deeply. And chosen some typical reservoirs in the Huixi-Liuhua, Huidong-Lufeng, Panyu4Sag and south Enping as comprehensive oil and gas reservoir anatomy targets. After that, the rules of oil and gas accumulation regularity in the Zhu I Depression vertically and laterally was summarized, oil and gas accumulation model in the different oil and gas accumulation zones was established and the main control factors of oil and gas accumulation was expounded.1. The static elements of hydrocarbon accumulationBased on the source rock pyrolysis data, began with sedimentary environment, organic matter abundance, types and maturity analyses, this research compared the characteristics of the two sets of effective hydrocarbon source rock (Wenchang group and Enping group) in Zhu I Depression:The sedimentary facies of Wenchang source rock is medium-deep lake and the major source of organic matter is algae. With the characteristics of high abundance, good types, high maturity, maily oil production, Wenchang source rock is the main hydrocarbon source rocks in the study area;The sedimentary facies of Enping source rock is shallow lake-lake-fluvial facies, and the major source of organic matter is higher plants. With the characteristics of high abundance, relatively poor type, low to medium maturity, and oil and gas production, the Enping source rock is also serves as important hydrocarbon source rocks in the study are. According to the core, logging, testing and a great quantity of other data, the reservoirs in Zhu I Depression are divided into five categories after the study of lithology, physical properties, pore types and structure, capacity and other factors of the reservoir:prolific oil layers mainly distributed in the reservoirs Ⅰ, Ⅱ and Ⅲ which have high porosity and permeability or medium to high porosity and permeability; reservoir Ⅳ has poor porosity and permeability, mainly with the found of poor oil and gas layer, oil and water layer, oily water layer and suspicious layer; IV category reservoir has extremely poor poroperm characteristics with few oil produced or no oil produced at all; The difference between the categories and qualities of the Neogene and Paleogene reservoirs is significant, the Neogene marine reservoir has high porosity and permeability, mainly belong to Ⅰ or Ⅱ reservoir categories, Hanjiang group and Zhujiang group are chief pay zones in the study area; The poroperm characteristics of Paleogene reservoir deteriorate obviously with lower proportion of Ⅰ, Ⅱ categories reservoirs, more Ⅲ, Ⅳ categories reservoirs, serve as secondary pay zones in the study area.Under the evaluation of the seal rock’s macroscopic distribution and microscopic quality (breakthrough pressure, the cumulative thickness, ratio of maximum thickness of single layer, mudstone ratio, etc.) several sets of regional seal rock in the Hanjiang group, upper Zhujiang group and Wenchang group in the study area are confirmed, with the features of widely spread, large thickness of mudstone, high breakthrough pressure, which can sealing the oil and gas effectively; The latter Zhujiang group and Enping group developed thick mudstone in most areas, which can be used as regional seal rock with the effectively ability of sealing; although the Zhuhai group is thin and sandy, it still has certain ability of sealing with the local seal rock developed. Controlled by the distribution of reservoir and seal rock, the study area developed three sets of reservoir-seal associations(upper middle Zhujiang group-latter Hanjiang group, Zhujiang group-Enping group, Wenchang group-former Palaeogene) vertically, at present most of the oil and gas distribution in the three reservoir-seal associations. Source rocks, reservoirs and seal rocks have matching relationship between spatial and temporal, which has formed three categories of source-reservoir-caprock associations (self sourced reservoirs, source rocks in the upper part and reservoirs in the lower part, associated with source rocks in the lower part and reservoirs in the upper part), and source rocks in the lower part and reservoirs in the upper part is the most common type found. Zhu Idepression has developed multiple types of traps widely, such as drape anticline traps, reverse drag anticline traps, tensile-tilting tectonic, fault nose and fault block, ect. Tectonic traps like faulted anticline are traps that found mostly prolific. Stratigraphic traps like lithology (such as sandstone lens), unconformity, buried hill, biological reef, ect., have great exploration potential, which is a fine exploration main battlefield of achieving new breakthrough in this area. However fault-lithologic trap and structure-lithology pinchout combination trap are more common in the Paleogene stratum.2. Dynamical conditions and process of hydrocarbon accumulationAccording to crude oil density and viscosity, the crude oil in ZhuIdepression could be divided into two types:The first kind of crude oil has low weight, low viscosity, belongs to the normal to light oil, distributed in most parts of Huizhou Sag and the other structures in sags else; The second category crude oil has high density, high viscosity with a considerable part of heavy oil or biodegraded oil, mainly distributed in Liuhua oilfield group and Panyu4Sag. Thanks to the indicating of biomarkers, Pr/Ph, C19/C23tricyclic terpane, double juniper alkanes T/αβC30hopane and C304-methyl/C29sterane were optimized as four effective parameters, after compared with hydrocarbon source rock, crude oil is classified into three types:category Ⅰ crude oil with "three low and one high"(low Pr/Ph and low C19/C23, low T/a(3C30hopane, high C304methyl/C29sterane) features, related to deep lake facies source rocks in Wenchang group, and play an important role to most of the oil and gas reservoirs in the Huixi-Liuhua region, Lufeng Sag, Panyu4Sag and Enping Depression; category Ⅲ crude oil has "three high one low"(high Pr/Ph, high C19/C23, low T/αβC30hopane, high C304-methyl/C29) characteristics, coming from the hydrocarbon source rocks of Enping group, currently only found in HZ9-2structure of Huizhou Sag; category II crude oil has relatively medium biomarker values between category I and II crude oil, regarded as a mix of Wenchang and Enping oil, distributed on part of oil pools in the convex ring around Huizhou Sag, Lufeng Sag and Panyu4Sag. Measured temperature data revealed that geothermal gradient in Zhu I Depression is between2.63-4.33℃/100m now, with an average of3.44℃/100m, belong to normally temperate zone overall. From the basin numerical simulation results, it can be seen that the ancient ground temperature field and heat flow in the study area is controlled by the mantle heat flow and tectonic movement change, with a fluctuate trend of high to low then gradually rising. The results also revealed that there is strong tectonic activity, high geothermal gradient and high heat flow during the chasmic stage in Pearl Mouth Basin; However, in the late chasmic stage, the fault activity weakened gradually with geothermal gradient and heat flow values reduced as well. Using the drilling test, AC well logging data, combining seismic prediction together, the distribution of formation pressure nowadays in Zhu I Depression is revealed. According to the results, The groups above Enping group has normal compaction with pressure coefficient of around1.0, belong to normal pressure zone; the top of Enping-Wenchang group developed low amplitude under compaction with pressure coefficient between1.0-1.2, belong to overpressure transition zone; The latter Wenchang formation developed under compaction widely with pressure coefficient exceeds1.4, belong to abnormal pressure zone, typical overpressure compartment. Gas chimney phenomenon identified from the three dimensional seismic data of Enping Depression also powerfully proved the existence of the overpressure in Wenchang group. Based on the two-dimensional numerical simulation technology, four hydrocarbon generation profiles of Huizhou, Lufeng, Panyu4Sag were selected to study the ancient stress field, the inversion results revealed that the formation pressure experienced a repeatedly progress of increase-decrease-increase, but generally showed an increasing trend as a whole, which illustrated complexity and overall consistency of pressure evolution in the Zhu I Depression. The oil and gas transportation systems in Zhu I Depression mainly belong to fault type, partly belong to unconformity and sandstone type (carbonate rock) conducting layer. Oil and gas migrated mainly through composite passage system which has various passage way combined rather than through single passage. Among passage systems, the fault-sand-fault passage system is the main passage for oil and gas migrate upward from source rock to the reservoir, it also is an important passage system for the gas and oil in the Huizhou Sag move to Dongsha uplift and fault-unconformity passage system play a significant role in oil and gas migration and accumulation in the latter stratums. With the help of reservoir fluid inclusion analysis, the research on inclusion microscopic observation, fluorescence spectrum analysis and determination of uniform temperature showed that the Zhu I Depression generally experienced two phases of oil and gas filling, in the medium Miocene epoch and the late Miocene epoch respectively, the later filling phase is the relatively more important one. From Wenchang group to the Zhujiang group, the two phase of oil and gas filling both have a trend of gradually late, the upper Zhujiang experienced only one phase of oil and gas filling, Dongsha movement is a critical period of hydrocarbon accumulation, many Neogene reservoirs were formed at this time, as many layers filled at the same with the later period. The master control factors of oil and gas accumulation regularity Typical reservoirs has been selected from different tectonic belt in Zhu I Depression, and its conditions and processed of hydrocarbon accumulation were comprehensive analyzed. Then, the formation mechanism and control factors of oil and gas reservoirs were discussed.3. Enrichment regularity and main controlling factorsOn the basis of this, the accumulation regularity of oil and gas in the study area are summarized vertically and laterally:On vertical, the oil and gas mostly gathered in the combination of latter Hanjiang group-upper Zhujiang group; On lateral, the oil and gas accumulated showed zonal differences, Huixi-Liuhua and Huidong-Lufeng are the best oil and gas accumulation zone in Huizhou Depression; Panyu4Sag has oil and gas distribute circularly around the generating hydrocarbon sag; However, oil and gas in Enping Depression are largely distribute in the bulge-fracture tectonic belt in the south. According to the petroleum source, passage system, hydrocarbon migration direction and pool-forming period, the modes of hydrocarbon accumulation in Huixi-Liuhua oil and gas accumulation zone could be sum up into four patterns:unisource-vertical migration-multiphase accumulation pattern, mixed source-vertical migration-multiphase accumulation pattern, mixed source-vertically&laterally migration-multiphase accumulation pattern and unisource-vertically&laterally migration-multiphase accumulation pattern; The hydrocarbon accumulation pattern in Huidong-Lufeng oil and gas accumulation zone is unisource-vertically&laterally migration-multiphase accumulation (fast); The hydrocarbon accumulation pattern in Panyu4Sag is unisource-vertical&lateral migration-single period accumulation pattern; The hydrocarbon accumulation pattern in south Enping Depression is unisource-vertically&laterally migration-single period accumulation pattern.After comprehensively analyzed the various factors of static element and dynamic conditions of hydrocarbon accumulation, the most important control factors of hydrocarbon accumulation in Zhu I Depression is illustrated:The degree of hydrocarbon accumulation is decided by the generating hydrocarbon depression, the scope of oil and gas migration is controlled by the passage system and the ability of oil and gas gathering is restricted by the tectonic position, reservoir conditions affect the size of the oil and gas reserves. |
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