| Author's Name: LIUZHU Ruizhi, HU Suyun, GU Zhidong, WANG Zecheng, LIU Guixia,
ZHAI Xiufen, JIANG Hua, ZHAO Rongrong, ZHANG Baomin, MO Wuling |
| The Sinian is extensively distributed in the Yangtze Block, South China, comprising the Doushantuo and
Dengying Formations from base to top. In the recent years, hydrocarbon exploration in the Sichuan Basin has revealed that
an approximately NW-SE trending intraplatform trough was formed during the Sinian, and its formation and evolution
controlled the reservoir development and gas field formation of the Dengying Formation around the trough. Now the
Anyue Field of the Dengying Formation with much research has been found in the eastern intraplatform trough. However,
the Sinian sequence, sedimentary evolution and reservoir characteristics in the western intraplatform trough still need to be
further studied. In this paper, two representative outcrops close to the western edge of the Sinian intraplatform trough, i.e.
the Dashuizha and Yanziyan sections in Mianzhu County, are selected to systematically analyze the stratigraphic division,
sedimentary evolution, and reservoir characteristics. The results of this paper show that: (1) The carbon isotope curves can
effectively distinguish the Sinian Doushantuo and Dengying Formations. The carbon isotope at the top of the Doushantuo
Formation is characterized by negative shift (up to -8‰), which is considerably compared with that at the base of the
Dengying Formation with positive shift. Both the Doushantuo and Dengying Formations were developed in the western
intraplatform trough. The Doushantuo Formation at least includes Members 2 to 4, and the typical cap dolomite of
Member 1 is not occurred. And the Dengying Formation is completely developed, from base to top, including Members 1
to 4. (2) The Doushantuo Formation is composed of a mixed succession including the lower dominant siliciclastic rocks
and the upper dominant carbonate rocks. Furthermore, the Doushantuo Formation is dominated by deep-water slope facies
deposition with the features of syn-depositional slump folds, rhythmites of interbedded carbonate and mudstone, and
olistostrome breccias. The Dengying Formation comprises dolomites with minor chert and mudstone, and it is mainly
shallow water carbonate platform facies deposition with the development of microbial laminated dolomites and botryoidal
dolomites. Thus, during the Sinian the study area experienced a sedimentary evolution from deep-water slope to shallow
water carbonate platform. (3) Thick microbial laminated dolomite reservoir with large scale and high quality was
developed in the Dengying Member 2 in the western intraplatform trough, which could be compared with eastern
intraplatform trough, revealing that the western intraplatform trough has also the excellent reservoir conditions of large gas
field formation. |
