| Author's Name: XIAO Kunye, ZHOU Hongpu, OU Yafei, CHEN Zhongmin, LIN Zimo, SUO Xiaofei,
CHEN Yajing, MA Xueying, ZHAO Ning |
| Carbonate rocks in Africa are characterized by extensive distribution along continental margins and scattered
occurrences in the continent. Carbonate rocks are primarily distributed in rift basins and continental margin rift basins
along the Tethyan margin of North Africa. Due to widespread marine transgressions during the Cretaceous and Cenozoic,
North Africa remained in a passive margin or epeiric sea environment, in favor of extensive carbonate deposition. They
also occur in passive continental margin basins of West and East Africa, although with limited continuity and thickness
due to narrow continental shelves and high fluvial input. Scattered carbonate deposits are found within intraplate rifts and
ancient cratonic basins, which are dominated by mixed clastic-carbonate sedimentation. Carbonate rocks are concentrated
in the Cretaceous and Cenozoic, with localized occurrences in the Jurassic, and extremely limited prior to the Paleozoic.
The accumulation conditions in African carbonate basins can be classified into three types: (1) The Sirte Basin and
Pelagian Basin have vertical stacking of mudstones, carbonates, and evaporites due to multiple cycles of rifting, inversion,
and sea-level fluctuations, indicating excellent petroleum systems in the Cretaceous and Cenozoic sequences. (2) The
Kwanza Basin and Lower Congo Basin of West Africa have petroleum assemblage of lacustrine source rocks, lacustrine
carbonates, and overlying evaporite seals, in addition, reservoirs with underlying lacustrine source rocks, overlying marine
carbonates and mudstones developed. (3) The Eratosthenes isolated platform generated biogenic reef due to the inherited
paleo-uplifts and suitable sea levels. There has an appropriate hydrocarbon system of Upper Cretaceous deep-sea source
rocks, reef carbonate reservoirs, and Miocene evaporite seals.of reservoirs affects pore development, distribution and reservoir quality. Relevant studies on the
Middle Triassic Leikoupo Formation in central Western Sichuan Basin are still blank. In order to reveal the diagenetic
evolution characteristics of dolomite reservoirs in the study area and their influence on reservoir quality, and to provide a
theoretical basis for the exploration of carbonate oil and gas, the study on the dolomite diagenesis of Leikou Formation in
western Sichuan Basin is systematically analyzed on the basis of data analysis such as core and thin section observations,
dolomite order degree analysis, fluid inclusions and carbon-oxygen stable isotope tests. The results show that: (1) The
Leikoupo Formation in central Western Sichuan mainly develops two different types of dolomite: micritic dolomite and
micritic algal clast dolomite. The low degree of ordering and low formation temperature of the dolomite crystals indicate
that they were formed by penecontemporaneous dolomitization. The reservoir spaces mainly consist of dissolved pores
developed along algal frameworks, intergranular pores, and tectonic breccia interstices. (2) The dolomite in the study area
has mainly undergone diagenetic processes such as fracturing, dolomitization, dedolomitization, dissolution, micritization,
cementation, and surface-induced demineralization. Among them, structural fracture and dissolution play an improving
role in the physical properties of the reservoir. Deep dissolution is the fundamental factor for the development of deep
secondary pores. (3) The correlation between the development characteristics of dissolution pores and structural breccia
and structural fractures is confirmed: acidic fluids were injected along the fracture space into the remaining pore
development areas such as sandy shoal and the framework of the algal layer to form secondary dissolution pores in the late
Indosinian stage. Vertical fissures and late structural breccia were formed in the early stage of the Himalayan Movement,
and late dissolution and calcite vein filling occurred. Horizontal fractures formed in the late Himalayan period, further
improving the physical properties of the reservoir. The research has for the first time clarified the diagenetic sequence and
pore evolution model of the dolomite reservoir of the Leikoupo Formation in central Western Sichuan Basin, and proposes
a three-stage reservoir control mechanism of "structural fracture-fluid dissolution-fracture modification", providing new
geological basis for the exploration of the Leikoupo Formation |
