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Hydrocarbon Occurrence Models Controlled by Graded Multi-factor in Marine Reservoirs, South China*
By
Sihuang Xu1, Yongsheng Ma2, Lianfu Mei3, Caiping Yuan3, and Tonglou Guo2
Search and Discovery Article #10118 (2006)
Posted November 20, 2006
*Adapted from extended abstract prepared for presentation at AAPG Annual Convention, Houston, Texas, April 9-12, 2006.
1China University of Geosciences, Wuhan, China, presently Kansas Geological Survey, Lawrence , KS
2South China Subsidiary of SINOPEC, Kunming, China
3China University of Geosciences, Wuhan, China
According to the concept of petroleum system, if a hydrocarbon pool is to exist, the geological elements including a petroleum source rock, reservoir rock, seal rock, and overburden rock, and the processes including trap formation and the generation-migration-accumulation of petroleum are essential (Magoon and Dow, 1994). These essential elements and processes must occur in space-time so that organic matter contained in a source rock can be converted to a petroleum pool. However, petroleum is a fragile mixture of oil and gas that is difficult to preserve (Blanc and Connan, 1994). The longer a petroleum accumulation exists in nature, the more likely the original petroleum in the accumulation and essential elements of the accumulation could be destroyed partly, or even totally.
geological history. It is still difficult to characterize the petroleum geology in terms of the essential elements of petroleum in South China because of the strong deformation. In order to depict petroleum geology effectively in such seriously deformed areas, the term pool-controlling factor is put forward in this paper. The pool-controlling factor is defined as the main geologic factor that controls the formation, destruction, preservation, and distribution of hydrocarbon pools. Unlike the essential elements of pool formation, the connotation of the pool-controlling factor widely indicates the reconstruction and preservation after formation of pools. After analysis of the complexity and hierarchy of pool- controlling factor in South China, this paper summarizes the occurrence models based on a graded multi-factor, taking South China as a whole petroliferous unit.
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Geological Setting for Marine Petroleum of South China South China is bounded on the north by Dabashan Plate Marginal Orogen, which separates it from the Qinling-Dabie Orogenic Belt, and the Tanlu Fault Belt, which separates it from North China Block. On the west South China is bounded by the Longmenshan and Yanyuan-Lijiang Plate Marinal Orogen, which separates it from Songpan Block and Sanjiang Orogenic Belt (Figure1). It is composed of the Yangtze Block and Cathaysian Block; they merged in late Sinian Period. Its tectonic evolution history may be divided into a stage of marine basins, extending from Sinian to middle Triassic with three tectonic cycles, and a nonmarine stage from late Triassic to Quaternary with four tectonic cycles. A series of significant tectonic movements occurred, with four dynamic types of tectonic basins, during its geologic history (Table 1). The tectonic framework controls sedimentary style and distribution of individual sequences. The Cambrian, Ordovician, Silurian, and Permian systems are distributed regionally, whereas other sequences only occur locally. Because of the complex and multi-period tectonism, the structural style and sedimentary characters differ from one place to another. South China is divided into sixteen structural subdivisions based on structural style, including Plate Marginal Orogens, Palaeozoic Craton Basins, Palaeo-uplifts, and Depressions (see Figure 1). The geological condition to form primary pools was excellent before the Indo-China movement during the Triassic, for the total thickness of marine strata from Sinian to Middle Triassic is as much as 10,000m. The secondary pools were formed by modification of primary pools since Indo-China tectonism. A number of fossil pools, remnant pools, normal oil and gas pools, CO2 pools, oil and gas seepages and bitumen on various scales are found in South China.
Complexity of Pool-Controlling FactorsIn South China, there are four sets of good and regionally distributed marine source rock (Cm1, O3-S1, P1, and P2), as well as five sets of average and local marine source rock (Cm2-3, O, S3, D, and T). These regional and local source rocks could have provided sufficient HC for primary accumulation. However, for most secondary accumulations in South China, the pool fluids did not come from regional or local source rocks. The following five types of individual and mixed-pool fluid source became very important pool-controlling factors in South China (Dai, 2003).
1) Gas cracked by crude oil. This crude oil might have been generated from kerogen or from previously accumulated oil in pools. The gas cracked from pool oil, named secondary gas, bears a direct relation to destruction of primary pools, while only indirectly related to source rock. It is the most common source of gas. 2) Gas dissolved in water. Former studies suggest that gas dissolved in water is an important pool gas source (Qiu, 1994). The principle in summary form is that original natural gas was dissolved early into formation water under high pressure, then came out of solution under lower pressure, and eventually migrated into traps. 3) Inorganic gas. It is reported that the CO2, of high purity and heavy carbon isotope composition (in Huangqiao Gas Field in lower Yangtze Block) came from the mantle, having migrated upward along deep basement faults. 4) Hydrocarbon generated from source rock. The oil and gas generated by regional and local source rocks are the primary type of HC, especially for fossil pools, such as Majiang fossil oil pool in Guizhou. 5) Hydrocarbon secondarily generated. Secondary generation took place in some areas that were uplifted during Indo-China and Yenshan movements and experienced renewed subsidence after the Yenshan movement; consequently Upper Cretaceous and Cenozoic sedimentary strata were deposited. It is thought that secondary generation occurred in localized source rocks within the Palaeozoic in the Yancheng depression in lower Yangtze Block, Lower Palaeozoic in the Chenhu area in middle Yangtze Block, Upper Palaeozoic in Shiwanshan fault-fold belt, and in the Cambrian in Chuxiong basin. 6) Mixed pool fluid source. It was composed of the preceding sources and is very common in South China.
Periods of Formation and Reconstruction Formation Periods: Mainly controlled by source-rock maturation and regional tectonic evolution, there are four petroleum formation periods in South China, including Caledonian, Indo-China, Yenshan and Himalayan. Some giant primary pools that formed during Caledonian were deformed into fossil pools, such as Majiang fossil pool in Guizhou and Yuhang fossil pool in Zhejiang. Jiannan gas field and Nandan reef fossil pool were formed during the Indo-China. During Yenshan and Himalayan periods, not only primary and secondary marine pools but also nonmarine pools were formed. Reconstruction Periods: There are five periods of pool reconstruction based on the statistics of oil and gas seepages and bitumen on the ground--Hercynian, Indo-China, early Yenshan, late Yenshan-early Himalayan, and late Himalayan periods. One pool could undergo one or more reconstruction periods continuously or intermittently. Because of multi-period of formation and reconstruction, the distribution of Palaeozoic-Mesozoic marine petroleum is complex and heterogeneous in space-time. Xiang-Qian-Gui block, Jiangnan uplift, and Yangtze block are richer in oil and gas seepage than elsewhere. Upper Palaeozoic, especially Permian, and Triassic, are richer in oil and gas seepage than Lower Palaeozoic and most of the Mesozoic (Zhao, 2002).
Preservation Condition for Petroleum The preservation condition varied in type and scale in South China. On a regional scale, tectonism is the basic preservation or dissipation condition of petroleum because of the impact on the continuity of evaporite seals and the nature and scale of fault systems. Subregionally, the preservation system for local petroleum is related to magmatism, faults, and cap rocks. Locally, geothermal field, fluid pressure, hydrochemistry, and even diagenesis affect the stability of hydrocarbon in pools.
Hierarchy of Pool-Controlling Factors Some factors control large regions, whereas others only control local areas. Pool-controlling factors can be classified by their controlling range. The larger the controlling range, the higher order the factor is. The pool fluid source was regarded as the highest category, category one (Table 2). Primary HC accumulation was mainly affected by regional source rock; secondarily generated HC and the gas dissolved in water was controlled by regional uplift and subsidence. Gas cracked from crude oil was affected by the regional thermal field and tectonism, and inorganic gas was obviously controlled by regional magmatism and fault systems. The relationship between fluid sources and the pools during the geological history are shown in Figure 2. The period of pool formation and reformation was category two. Petroleum accumulation can be classified as primary and secondary. Primary accumulation means pool fluid came specifically and directly from source rock--not from another pool; in other words, petroleum migrated directly from source to trap. Secondary accumulation means pool fluid is derived from another pool; that is, from pool to new trap. Secondary accumulation always accompanied the reconstruction and deconstruction of other pools. The spatial distribution of pool formation and its later reconstruction may be regional or local, but it is usually smaller than that of pool fluid source. Examples of periods of reconstruction and elements/areas affected are as follows:
The accumulation elements are category three, including preservation condition, carrier system and migration pathway, trap types and reservoir, and cap rock. On a different scale there are different types of preservation conditions. However, for an individual pool of hydrocarbon, the direct preservation factor usually is local.
Pool Models Controlled by Graded Multi-Factors Principles for Setting up the Models Accordingly to the above statements, it is clear that the pool-controlling factors for marine petroleum in South China are complex and graded. To set up the pool models based on graded multi-factors, two principles are needed to be followed: (1) Principle of Large Scale High Category: The larger the scale of the study area, the higher the involvement of order of pool-controlling factors. When we study South China as a whole geological unit, the top category factors must be used, generally including category one and two. If a local problem is studied, only lower categories, such as three or four, are needed. (2) Naming principle: The name of model includes the pool-controlling factors, from higher to lower category.
Based on category one and two, the pool fluid source, and the period of pool formation and reconstruction, ten types of petroleum models in South China have been recognized. Four of them are regarded as typical cases; they are type Well Mian31, type Zhujiadun, type Huangqiao CO2 field, and type Sichuan. As yet others are not found to typical cases (Table 3). In Sichuan, as an example, its pool fluid source is typically mixed gas, composed primarily of the gas cracked secondarily from crude oil and the gas generated late by source rock, with only minor amounts of gas that was dissolved in water. Existing gas pools are reformed from pre-existing oil and gas pools. The deformation and secondary accumulation took considerable time and were multi-phased. The last period was the main formation period. In Sichuan basin, some large gas pools (e.g., Sinian system gas pool in Weiyuan Gas Field, Carboniferous gas pool in Wubaiti Gas Field, and Jiannan Gas Field) belong to this type. The model with mixed gas source and secondary accumulation is a very common type in South China. It should be an important target for future exploration. If three categories of pool-controlling factors were considered, then more types of models in South China could be established. With the Sinian system gas pool in Weiyuan Gas Field as an example, if the trap type, one of category three factors, is considered, then the pool model will be mixed gas source and late secondary accumulation and anticline pool model (Figure 3).
1) The pool-controlling factor is defined as the main geologic factor that controls the formation, destruction, preservation, and distribution of hydrocarbon pools. It is especially applicable to the strongly deformed area. 2) Marine petroleum in South China was mainly controlled by graded multi-factors. The first category of pool-controlling factor is pool fluid source; the second category is period of pool formation and reconstruction; the third is accumulation elements including preservation condition and trap types. 3) Ten types of marine petroleum models of South China are recognized, based on categories one and two of the pool-controlling factors. Four of the models have been found. Type Sichuan was the most important model for marine petroleum in South China. 4) The principle, that pools are controlled by graded multi-factors, has a significant impact on the study of the formation and distribution of marine petroleum in South China, as well as other similar areas.
Magoon, L.B., and W.G. Dow, 1994, The petroleum system, in L.B. Magoon and W.G. Dow, eds., The petroleum system-from source to trap: AAPG Memoir 60, p. 3-24. Blanc, Ph., and J. Connan, 1994, Preservation, degradation, and destruction of trapped oil, in L.B. Magoon and W.G. Dow, eds., The petroleum system-from source to trap: AAPG Memoir 60, p. 237-247. Dai, Shaowu, et al., 2001, Thinking of Meso-Palaeozoic hydrocarbon exploration in South China: Oil and Gas Geology, v. 22, p.195-202. Qiu, Yunyu, et al., 1994, Pool-forming model for Weiyuan Gas Field: Natural Gas Industry, v. 14, p. 9-13. Zhao, Zongju, et al., 2002, Factors controlling the formation and evolution of Mesozoic and Palaeozoic oil/gas reservoir and their exploration targets in South China: Natural Gas Industry, v. 22, no.5, p. 1-6. |
