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2008 Vol. 19, No. 6

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Geodynamic Indications on Igneous Rocks of Transitory Geologic Setting during the Late Hercynian in North Tarim Basin
Liangjie Tang, Taizhu Huang, Zehong Cui, Suju Yang, Yixiu Zhu, Xianmei Jin
2008, 19(6): 577-590.

Basic and intermediate-acidic igneous rocks were developed during the late Hercynian in North Tarim basin. The geochemistry characteristics of the rocks show that basic volcanic rock has K2O/Na2O = 0.18-0.61 < 1 and falls into a category of basalt of sodium system. The rocks contain en- riched large-ion lithophile elements (LILE) (K, Rb, Ba, Th) and high-field strength elements (HFSE) (Nb, Ta, Ti, Zr, P), with the magmatic material from the upper mantle. The intermediate-acidic volcanic rocks have σ = 1.91-2.96 < 3.3, K2O/Na2O = 1.25-1.59 > 1, as well as the enriched LILE and depleted HFSE (Nb, Ta, Ti, P), presenting the same trace element compositions and character- istics as in the granitic rocks of South Tianshan Mt.; they are either shoshonitic igneous rocks or high-K calc-alkaline igneous rocks, with a distinct crust-derived component feature. The comprehensive analyses on the characteristics of the trace elements, the graphic tectonic dis-crimination, and the distribution features of the two types of igneous rocks show that they were formed under different tectonic settings and geodynamic environments: the basalt was formed in the active rifting period when the active mantle upwelling caused the thinning of lithosphere; the intermediate- acidic volcanic-intrusive rock was formed in the island arc area of the active continental margin in North Tarim; the formation is associated with the plate subduction during the course of South Tian- shan Ocean closure-the subduction of Middle Tianshan Mountain toward the Tarim plate. The basic and intermediate-acidic igneous rocks reveal a tectonic regime of extension-extrusion transition, which is significant in determining the key tectonic revolution period of North Tarim basin.

Evidence of Thermal Evolution History of Northeast Sichuan Basin—(U-Th)/He Low Temperature Thermochronometry of Apatite and Zircon
Jianzhong Qin, Jie Wang, Nansheng Qiu
2008, 19(6): 591-601.
Segment, Linkage, and Extensional Fault-Related Fold in Western Liaodong Bay Subbasin, Northeastern Bohai Sea, China
Minghui Yang, Xinhuai Zhou, Gang Wei, Le Liu, Chunxia Li, Xiaofeng Zheng, Xiaojian Liu, Libao Gao
2008, 19(6): 602-610.
Fault Characteristics in Longmen Mountain Thrust Belt, Western Sichuan Foreland Basin, China
Guimei Wan, Liangjie Tang, Keming Yang, Wenzheng Jin, Zhizhou Lü, Yixin Yu
2008, 19(6): 611-624.
Effect of Faulting on Ordovician Carbonate Buried-Hill Reservoir Beds in Hetianhe Gas Field, Tarim Basin
Xiuxiang Lü, Zhongkai Bai, Jianjiao Li, Weiguang Wang, Hui Fu, Qinghua Wang
2008, 19(6): 625-635.
Water Soluble Gas in Deep Carbonate Reservoir, Sichuan Basin, Southwest China
Guozhi Wang, Shugen Liu, Wenchao Su, Wei Sun, Dong Wang, Haifeng Yuan, Guosheng Xu, Can Zou
2008, 19(6): 636-644.

Based on temperature and pressure of fluid inclusion, phase of organic inclusion in calcite and quartz filled in vug in the deep carbonate reservoir and the natural gas composition in Weiyuan (威远) gas field in Sichuan (四川) basin, research indicates that water soluble gas exists in deep carbonate reservoir, which reconstructs development and effusion process of water soluble gas. The overpressure formed during oil thermal cracking can reach 105-170 MPa in Sinian and Cambrian reservoir in Central Sichuan and 78-86 MPa in Cambrian reservoir in Southeast Sichuan. The high temperature caused by deep burial and overpressure caused by thermal cracking make thermal cracking gas dissolve in water so that it becomes water soluble gas. The ratios of gas to water can reach 50-90 m3/m3 and 10-30 m3/m3, respectively, in deep carbonate reservoir in Central and Southeast Sichuan. Methane dissolving in water exists in form of liquid phase. Until now, the decreases in temperature and pressure due to the uplift during 74 Ma make water soluble gas separate from water, water soluble gas pool or mixed gas pool of thermal-cracking gas and water soluble gas are modified or even destroyed in varying degrees.This may be the case of Weiyuan gas field.

Main Controlling Factors on Hydrocarbon Accumulation and Distribution in Marine Sedimentary Sequences in South China
Liguo Cai, Yan Zhou, Zhenrui Bai
2008, 19(6): 645-655.

Multiple source rock assemblages were deposited in the sedimentary provinces in South China in geologic history, and some of them were destructed by and some survived against multiple tectonic movements. Therefore, multiple sources, mixed sources, and uneven distribution of sources occurred in the marine sedimentary basins in South China during the late stage of hydrocarbon pooling. Epidiagenesis of the marine carbonate reservoirs and its modification to reservoir poroperm character- istics determined the formation and the scale of natural gas pools. The exploration practices show that the large to medium gas fields mainly occur in areas with high-quality reservoirs. Detailed study of the paleo-oil accumulations and typical oil and gas reservoirs reveals that the basins experienced multi- phase superimposition and modification, leading to the distribution of the Paleozoic paleo-oil accumu- lations and bitumen in the peripheral areas. The phenomenon that oil and gas production concentrates in the Sichuan basin indicates that the overall sealing conditions of a basin determine the oil/gas poten- tials and the scale of oil and gas production. This is a critical factor controlling the accumulation and distribution of gas in the marine sequences in South China. The early oil and gas pools in the Yangtze platform left billions of bitumen in the peripheral areas due to the destruction of seals. Since the Hima- layan, "late-generation and late-accumulation" gas pools represented by the gas pools in the Sichuan (四川) basin were formed in the marine sedimentary sequences in South China as a result of the change of the sealing conditions. Current gas discoveries appear to be "paleo-generation and paleo- accumulation" gas pools but actually are "late-generation and late-accumulation" gas pools. These pat- terns of hydrocarbon pooling clearly depict themselves in western Sichuan basin and Weiyuan (威远) gas field. It is revealed that the gas pools in the Sichuan basin were mainly formed as a result of hydro- carbon phase change (thermal cracking of oil to gas), miscible migration, and dynamic equilibration since the Himalayan. A large number of gas pools were formed in the Himalayan and the gas pools in the marine sequences are characterized by late pooling; this kind of gas fields/pools are controlled by: (1) effectiveness of modification and superimposition of the marine basins, (2) effectiveness of the source rocks, (3) effectiveness of the overall preservation conditions, and (4) effectiveness of plays.

Sequential Extraction on Oil Sandstones from TZ401 Well—A Case Study on Filling History of Hydrocarbon Reservoir
Changchun Pan, Dayong Liu
2008, 19(6): 656-664.

Sequential extraction was performed on two oil sandstones from the Upper Carbonifer- ous oil columns of TZ401 well. The free oils of these two oil sandstones and a crude oil from the Lower Carboniferous oil column of this well have low ratios of C28/ (C27 + C28 + C29) steranes and gammacer- ane/C31 hopanes, ranging of 0.11–0.16 and 0.09–0.15, respectively, similar to those from the Middle–Upper Ordovician source rock. However, these two ratios for the adsorbed and inclusion oils of these two oil sandstones are relatively high, ranging of 0.29–0.31 and 0.26–0.40, respectively, similar to those of the Cambrian–Lower Ordovician source rock. This result demonstrates that the initial oil charging the reservoirs was derived from the Cambrian–Lower Ordovician source rock, whereas the later charging oil was derived from the Middle–Upper Ordovician source rock.

Formation Models and Distribution of Oil and Gas Pools in Tarim Basin, China
Shengxiang Long
2008, 19(6): 665-674.

This article reports the main formation models and distribution of the oil and gas pools in Tarim basin, China, including (1) occurrence of the found oil and gas pools, (2) main formation models of oil and gas pools, and (3) distribution law of oil/gas pools. Petroleum is distributed widely in the strata of Tarim basin from the Sinian at the bottom to the Neogene at the top. However, the found oil and gas fields are mainly distributed in Shaya (沙雅) uplift, Tazhong (塔中) uplift, and Kuche (库车) depression. This article presents 4 main formation models, namely, early formation and long-term preservation, early formation and late reformation, middle–late multiphase-multisource formation, late single-stage formation. Tarim basin is very rich in petroleum resources. Long-term inherited intrabas- inal paleohighs and slope zones are the most favorable areas for accumulation of hydrocarbons, but the types of oil and gas pools are different from area to area. The control of unconformities and faults on hydrocarbon accumulating is prominent in Tarim basin. Preservation conditions are of utmost impor- tance. Formation of some oil and gas pools is the result of reforming and re-accumulating of early accumulated hydrocarbons.

Mechanism of Secondary Pore Formation and Prediction of Favorable Reservoir of Paleogene in Jiyang Sag, Eastern China
Xiaomin Zhu, Huanqing Chen, Dakang Zhong, Qin Zhang, Shanwen Zhang, Xixue Lü
2008, 19(6): 675-684.

Jiyang (济阳) sag is an oil rich basin, consisting of Huimin (惠民), Dongying (东营), Zhanhua (沾化), and Chezhen (车镇) depressions. The clastic rock of Paleogene has undergone early and middle diagenetic stages and now the main clastic reservoir is in the middle diagenetic stage. Primary and secondary pores are developed in Paleogene sandstone, the latter is generated from the dissolution of feldspar and calcite cement in rocks owing to the organic acid from the maturated source rock, but the materials dissolved are different in different depressions. The reservoir secondary pores of Dongying depression are generated from the dissolution of calcite cement, the ones of Zhanhua and Huimin depressions from the dissolution of feldspar, the secondary pores of Chezhen depression from the dissolution of feldspar in upper section, and the dissolution of calcite cement in the lower section of Paleogene, respectively. The secondary pores are developed in two depths and the depth goes down from west to east, from south to north in Jiyang sag. The major controlling factors for secondary pore development are maturity and location of source rock. Lastly, the favorable reservoirs are evaluated according to reservoir buried depth, sedimentation, and diagenesis. The reservoir with high quality is located in the northern and central parts in Dongying depression; there are some good reservoirs in Gudao (孤岛), Gudong (孤东), and Gunan (孤南) areas in Zhanhua depression, and the favorable res- ervoirs are located in the north steep slope and the south gentle slope of Chezhen depression and cen- tral uplift, south gentle slope of Huimin depression.

Hydrocarbon Accumulation Mechanism of Sinian Reservoir in Anpingdian-Gaoshiti Structure, Middle Sichuan Basin
Guosheng XU, Min MAO, Haifeng YUAN, Shugen LIU, Guozhi WANG, Cunjian ZHOU
2008, 19(6): 685-699.

The Sinian reservior in Anpingdian (安平店) -Gaoshiti (高石梯) structure,Middle Si-chuan (四川) basin,is of great importance to prospect for oil and gas. This article dissects the hydrocarbon accumulation mechanism of this area on the basis of comprehensive methods of organic geochemistry,fluid inclusion,modeling of hydrocarbon generation and expulsion from source rocks,and by combining structure evolutions and analyzing the key geologic features of hydrocarbon origin and trap. According to the fluid inclusion homogenization temperature analysis,there exist at least three stages of fluid charging in the Sinian reservoir. From Middle–Late Jurassic to Early Cretaceous,oil cracked to gas gradually owing to high temperature at 200–220 ℃. The Sinian gas pool was mainly formed at the stage when natural gas in trap was released from water and paleo-gas pools were being adjusted. It was a process in which natural gas dissipated,transferred,and redistributed,and which resulted in the present remnant gas pool in Anpindian-Gaositi tectonic belt. The authors resumed such an evolution process of Sinian reservoir as from paleo-oil pools to paleo-gas pools,and till today's adjusted and reconstructed gas pools.

Energy Field Adjustment and Hydrocarbon Phase Evolution in Sinian–Lower Paleozoic, Sichuan Basin
Shugen LIU, Hua WANG, Wei SUN, Guozhi WANG, Guosheng XU, Haifeng YUAN
2008, 19(6): 700-706.

The Sinian-Lower Paleozoic (also called the lower association) in Sichuan (四川) basin has undergone geologic evolution for several hundred million years. The subsidence history of the Sinian-Lower Paleozoic can be divided into four stages: the stable subsidence during Cambrian and Silurian; the uplift and denudation during Devonian and Carboniferous; the subsidence (main process) during Permian to Late Cretaceous; and the rapid uplift and denudation since Late Cretaceous. The later two stages could be regarded as critical factors for the development of oil and gas in the lower association. The evolution of energy field such as temperature,pressure,and hydrocarbon phase in the lower association during the deep burial and uplift in the third stage might be induced as follows: (1) super-high pressure was developed during oil-cracking,previous super-high pressure was sustained,or changed as normal pressure during late uplift; (2) temperature increased with deep burial during persistent subsidence and decreased during uplift in late stage; (3) as a response to the change of the energy field,hydrocarbon phase experienced a series of changes such as organic material (solid),oil (liquid),oil-cracking gas (gaseous) + bitumen (solid) + abnormal high pressure,gas cap gas with super-high pressure (gaseous) + bitumen (solid) + water soluble gas (liquid),and gas in pool (gaseous) + water soluble gas (liquid) + bitumen (solid). The restoration of hydrocarbon phase evolution is of important value for the exploration of natural gas in the Sinian-Lower Paleozoic in Sichuan basin.

Paleo-temperature Evolution and Water Soluble Gas in Sinian Reservoir, Anpingdian-Gaoshiti Structural Zone, Central Sichuan Basin
Haifeng YUAN, Guosheng XU, Shugen LIU, Guozhi WANG
2008, 19(6): 707-714.

The paleo-temperature evolution of Sinian reservoir of Anping (安平) 1 well was rebuilt by taking the method of apatite fission track and Easy%Ro model. The result of apatite fission track determines the accurate burial history and overcomes the flaw that the vitrinite reflectance is taken as paleo-temperature indicator simply. The authors used the laser Raman technique to analyze the meth- ane present in the calcite and quartz fluid inclusions of Sinian reservoir,finding that the methane is water soluble gas. The authors also simulated the paleo-pressure of fluid inclusion by using PVTsim software and finally worked out the methane solubility in water.

Paleo-oil-Water Contact and Present-Day Gas-Water Contact: Implication for Evolution History of Puguang Gas Field, Sichuan Basin, China
Pingping LI, Huayao ZOU, Yuanchun ZHANG, Cunwu WANG, Xuefeng ZHANG
2008, 19(6): 715-725.

The Puguang (普光) gas field is the largest gas field found in marine carbonate in China. The Puguang gas field experienced complicated evolution history from paleo-oil pool to gas pool. The purpose of this article is to reveal the evolution history of Puguang gas field through systematic study on the relationship between paleo-oil-water contact (POWC) and present-day gas-water contact (PGWC). POWC was recognized by observing the change of relative content of residual solid bitumen in the cores,and PGWC was observed using log and drilling stem test data. Two types of relationship between POWC and PGWC were observed in the Puguang gas field: POWC is above PGWC,and POWC is below PGWC. The former is normal as oil cracking may cause gas-water contact to move downward. The latter can be interpreted by lateral gas re-migration and re-accumulation caused by changes in structural configuration. The relationship between POWC and PGWC suggests that during oil charge,the southwestern and northwestern parts of the Puguang gas field were structurally lower than the northeastern and southeastern parts. Thrusting from Xuefengshan (雪峰山) since Yanshanian movement and from Dabashan (大巴山) since Himalayan movement resulted in the relative uplift of the southwestern and northwestern parts of the Puguang structure,which significantly changed the structural configuration. Based on the paleo-structure discussed in this article,the most probable migration directions of paleo-oil were from the northwest to the southeast and from the southwest to the northeast. Consequently,the evolution history of the Puguang gas field can be divided into three stages,namely,oil charging (200-170 Ma),cracking oil to gas (155-120 Ma),and gas pool adjustment (12-0 Ma).

Reserve and Pressure Change of Paleo-oil Reservoir in Puguang Area, Sichuan Basin
Yuanchun ZHANG, Huayao ZOU, Cunwu WANG, Pingping LI
2008, 19(6): 726-738.

The Puguang (普光) gas field is the largest gas field found in marine carbonates in China. The Feixianguan (飞仙关) and Changxing (长兴) reservoirs are two such reservoirs that had been buried to a depth of about 7 000 m and experienced maximum temperature of up to 220 ℃ before uplift to the present-day depth of 5 000-5 500 m,with present-day thermal maturity between 2.0% and 3.0% equivalent vitrinite reflectance (Ro). Bitumen staining is ubiquitous throughout the Feixianguan and Changxing formations,with the greatest concentrations in zones with the highest porosity and permeability,suggesting that the solid bitumen is the result of in-situ cracking of oil. According to the distribution of bitumen in the core,the paleo-oil boundary can be approximately determined. The paleo-oil resource is calculated to be about (0.61-0.92) × 10^9 t (average 0.76 × 10^9 t),and the cracked gas volume is about (380.80-595.80) × 10^9 m^3 (average 488.30 × 10^9 m^3); at least 58.74% of cracked gas is preserved in Puguang gas field. The study area experienced not only the cracking of oil but also thermochemical sulfate reduction,resulting in large quantities of nonhydrocarbon gas,with about 15.2% H2S and 8.3% CO2,together with the structural reconfiguration. During the whole process,the great change of volume and pressure compels the PVTsim modeling software to simulate various factors,such as the cracking of oil,the thermochemical sulfate reduction (TSR) and the tectonic uplift in both isolated and open geological conditions,respectively. The results show that although any one of these factors may induce greater pressure changes in an isolated system than in a closed system,the oil cracking and C3+ involving TSR lead to overpressure during the early stage of gas reservoir. Therefore,the tectonic uplift and the methane-dominated TSR,as well as the semi-open system contribute to the reducing pressure resulting in the current normal formation pressure.