| Author's Name: ZHENG Jianfeng1,2,3, BAI Xuejing 3,4, DAI Kun5, HONG Shuxin3,4, LIU Yunmiao3,4,
DUAN Junmao1,2,3, GE Zhidan3,4, LIU Lianjie3,4 |
| The Maokou Formation of Middle Permian has huge resource potential and is an important target for natural
gas exploration in Sichuan Basin. In recent years, significant exploration breakthroughs have been made in the dolomite of
the 2nd member of Maokou Formation in central Sichuan Basin, and the gas production of several wells has exceeded one
million cubic meters, which reveals the huge exploration prospect in this field. However, the genesis of dolomite is still
unclear, which restricts the prediction of dolomite distribution. Focusing on the core exploration wells in Hechuan area, a
detailed description of the petrological characteristics based on core and thin sections is carried out, and representative
samples of dolomite and limestone are selected for carbon oxygen isotope, strontium isotope, rare earth element, and UPb dating analysis. Taking into account the geological background, it was clarified that: (1) Dolomite is mainly developed
in the middle-upper part of the 2nd member of Maokou Formation, with a thickness of 1-25 m, and its original rock is
grainy limestone. (2) The dolomitization fluid is mainly seawater, and dolomitization occurred in the quasicontemporaneous period-early burial period. (3) The shoal developed in a relatively paleogeomorphologic high part of the
2nd member of Maokou Formation was susceptible to syngenetic karstification, and a large fracture-cavern system
developed in the phreatic zone. Fracture-cavern system were filled with bioclastic particles, marl and Mg2+ rich seawater,
and dolomitization occurred during the shallow burial process. Based on the new research results of dolomite genesis, it is
clear that the paleogeomorphologic high part is the favorable area of dolomite of the 2nd member of Maokou Formation,
which provides a basis for the prediction of dolomite reservoir distribution in the study area and effectively guides the
exploration deployment.
Differences in the characteristics and distribution patterns of dolomite reservoirs of the Upper Cambrian Xiaqiulitage
Formation in the western Tarim Basin. Based on a systematic analysis of rock thin section, carbon and oxygen isotope
compositions, and U-Pb dating, the conclusions are drawn as follows: (1) The Xiaqiulitage Formation, with a total
thickness of 350 m, is divided into six members, and is composed of grain dolomite, thrombolite dolomite, stromatolite
dolomite, and laminated microbialite dolomite. Seven lithofacies association and two third-order sequences are identified
in the Xiaqiulitage Formation, reflecting the overall transition of tidal flat subfacies to inner platform shoal subfacies from
bottom to top. (2) The reservoir spaces are dominated by matrix dissolution pores, vugs (dissolution cavities), and
intergranular fractures within breccias. The columnar stromatolitic dolomite and thrombolitic dolomite exhibit the best
physical properties, followed by grain dolomite, with the overall characteristics of moderate-to-high porosity and
moderate-to-low permeability. A comprehensive evaluation indicates that the reservoir properties are optimal in Member
1, Member 2, and Member 6, while Member 5 ranks slightly lower. (3) The dolomite was formed during the early
diagenetic stage, and reservoir development is primarily controlled by the combined effects of sedimentary microfacies,
unconformity surfaces, and high-frequency sequences. The reservoirs can be classified into two types: unconformity-karst
dolomite reservoirs and inner mound-shoal dolomite reservoirs. This research provides critical support for evaluating
favorable exploration zones in the Cambrian dolomite plays of the western Tabei area, and offers reliable evidence for
hydrocarbon reservoir assessment, particularly in the Xiongying region. |
