Three Episodes of Hydrocarbon Generation and Accumulation of Marine Carbonate Strata in Eastern Sichuan Basin,China

It is concluded that there are three hydrocarbon generation and accumulation processes in northeastern Sichuan on the basis of the characteristics of solid bitumen, gas-light oils-heavy oils, homogenization temperature of fluid inclusions and diagenesis for beach- and reef-facies dolomite gas- bearing reservoirs in the Puguang Gas Field, northeastern Sichuan Basin, southern China. The first hydrocarbon generation and accumulation episode occurred in the Indosinian movement （late Middle Triassic）. The sapropelic source rocks of the O3w （Upper Ordovician Wufeng Formation）-S1l （Lower Silurian Longmaxi Formation） were buried at depths of 2500 m to 3000 m with the paleogeothermal temperature ranging from 70℃ to 95℃, which yielded heavy oil with lower maturity. At the same time, intercrystalline pores, framework pores and corrosion caused by organic acid were formed within the organic reef facies of P2ch （Upper Permian Changxing Formation）. And the first stage of hydrocarbon reservoir occurred, the level of surface porosity of residual solid bitumen {solid bitumen/ （solid bitumen ＋ residual porosity）} was higher than 60%. The second episode occurred during the Middle Yanshanian movement （late Middle Jurassic）. During that period, the mixed organic source rocks were deposited in an intra-platform sag during the Permian and sapropelic source rocks of O3w-S1l experienced a peak stage of crude oil or light oil and gas generation because they were buried at depths of 3500 m to 6800 m with paleogeothermal temperatures of 96-168℃. At that time, the level of surface porosity of residual solid bitumen of the T1f shoal facies reservoirs was between 25% and 35%, and the homogenization temperatures of the first and second stages of fluid inclusions varied from 100℃ to 150℃. The third episode occurred during the Late Yanshanian （Late Cretaceous） to the Himalayan movement. The hydrocarbon reservoirs formed during the T1f and P2ch had the deepest burial of 7700 m to 8700 m and paleogeotemperatures of 177℃ to 220℃. They could be cracked into dry gas （methane）, and the same with the source rocks of the Permian and O3w-S1l because they all reached the pyrolysis stage under such conditions. Consequently, the present natural gas （methane） reservoirs were developed.

Type division and controlling factor analysis of 3rd-order sequences in marine carbonate rocks

Type division and controlling factor analysis of 3rd-order sequence are of practical significance to tec-tonic analysis, sedimentary environment identification, and other geological researches. Based on the comprehensive analysis of carbon and oxygen isotope trends, paleobathymetry and spectral-frequency of representative well logs, 3rd-order sequences can be divided into 3 types： （a） global sea level （GSL） sequence mainly controlled by GSL change;（b） tectonic sequence mainly controlled by regional tectonic activity;and （c） composite sequence jointly controlled by GSL change and regional tectonic activity. This study aims to identify the controlling factors of 3rd-order sequences and to illustrate a new method for classification of 3rd-order sequences of the middle Permian strata in the Sichuan Basin, China. The middle Permian strata in the Sichuan Basin consist of 3 basin-contrastive 3rd-order sequences, i.e., PSQ1, PSQ2 and PSQ3. Of these, PSQ1 is a GSL sequence while PSQ2 and PSQ3 are composite sequences. The results suggest that the depositional environment was stable during the deposition of PSQ1, but was activated by tectonic activity during the deposition of the middle Permian Maokou Formation.

Discovery of Early Proterozoic Carbon Isotope Shifts

The authors have detailedly and systematically studied the carbon isotopic composition of Early Proterozoic carbonate rocks. Samples which are all dolomicrite were taken from the Jianancun, Daguandong and Huaiyincun Formations of the Hutuo Group in Wutai County Shanxi Province, North China. A total of 209 samples were analysed for their carbon isotope compositions, and the mean sampling interval was 6.9 m. Carbon isotope analysis clearly shows δ13C shifts at the boundary between the Jian'ancun Formation and Daguandong Formation and near the boundary between the Daguandong Formation and Huaiyincun Formation. Like carbon isotope shifts at the Cretaceous-Tertiary, Permian-Triassic and Precambrian-Cambrian boundaries, the discovery of δ13C shifts in the Early Proterozoic has important significance in further studies on Early Proterozoic biotic evolution, regional and global stratigraphic correlation, and carbon geochemical cycles.

Seismic Acquisition Parameters Analysis for Deep Weak Reflectors in the South Yellow Sea

The Mesozoic-Paleozoic marine residual basin in the South Yellow Sea(SYS) is a significant deep potential hydrocarbon reservoir. However, the imaging of the deep prospecting target is quite challenging due to the specific seismic-geological conditions. In the Central and Wunansha Uplifts, the penetration of the seismic wavefield is limited by the shallow high-velocity layers(HVLs) and the weak reflections in the deep carbonate rocks. With the conventional marine seismic acquisition technique, the deep weak reflection is difficult to image and identify. In this paper, we could confirm through numerical simulation that the combination of multi-level air-gun array and extended cable used in the seismic acquisition is crucial for improving the imaging quality. Based on the velocity model derived from the geological interpretation, we performed two-dimensional finite difference forward modeling. The numerical simulation results show that the use of the multi-level air-gun array can enhance low-frequency energy and that the wide-angle reflection received at far offsets of the extended cable has a higher signal-to-noise ratio(SNR) and higher energy. Therefore, we have demonstrated that the unconventional wide-angle seismic acquisition technique mentioned above could overcome the difficulty in imaging the deep weak reflectors of the SYS, and it may be useful for the design of practical seismic acquisition schemes in this region.