Abstract
Abstract Paleogene lacustrine dolomicrite of Unit II of the lower Xingouzui Formation in the southern part of the Qianjiang Depression, China forms “tight carbonate” hydrocarbon reservoirs that can be divided into the following four dolomicrite types: dolomicrite, muddy dolomicrite, sandy dolomicrite, and gypsiferous dolomicrite. These four dolomicrite types occur in the following three distinct combinations, which are referred to as subfacies: (1) interbedded dolomicrite (or sandy dolomicrite) and dark-gray siliciclastic mudstone, interpreted as open lacustrine deposits, (2) grayish-dark siliciclastic mudstone with thinly-bedded muddy dolomicrite or gypsiferous dolomicrite, interpreted as restricted lacustrine deposits, and (3) grayish-green mudstone with thinlybedded brown-yellow siltstone, interpreted as lacustrine shoreline deposits. The spatial distribution of these subfacies was controlled by the paleo-provenance system, paleo-microgeomorphology, and lake level fluctuations. In the open lacustrine subfacies, dolomicrite or sandy dolomicrite are often vertically interbedded with dark argillite rocks, forming a suitable petroleum source rockreservoir association. The dolomicrite and sandy dolomicrite reservoirs possess many secondary crystalline pores and few dissolution pores, with mesopore porosities in the range of 15%–25%. The dark mud shale interval has total organic carbon (TOC) values exceeding 4% (averaging 1.29%). The organic matter is mainly type II2, with vitrinite reflectance values averaging 0.57, which indicates potential for the generation of immature oil. The oil-source correlation results indicated that the n-alkanes of the crude oil and source rock samples from Unit II of the lower Xingouzui Formation exhibit single peak values, with a main peak (post-peak) in C22 and a Pr/Ph ratio $#x003C; 1 in a C2920R $#x003E; C2720R $#x003E; C2820R distribution. These features indicate that source-reservoir interbedding is characteristic of selfsourced reservoirs. The interbedded dolomicrite (or sandy dolomicrite) and dark-gray siliciclastic mudstone, i.e., subfacies (1), is a favorable target for hydrocarbons in the study area.
Highlights
In China, restricted lacustrine carbonate deposits are present in Permian, Cretaceous, lower Paleogene, and upper Neogene strata [1]
This study addresses some of these unresolved issues by presenting a study of dolomicrite in the Paleogene Xingouzui Formation in the southern Qianjiang Depression (Figure 2)
Previous interpretations of the Xingouzui Formation suggest that this unit consists of lacustrine deposits that accumulated in an arid to semi-arid climate [42, 44, 45]
Summary
In China, restricted lacustrine carbonate deposits are present in Permian, Cretaceous, lower Paleogene, and upper Neogene strata [1]. In the Kuche Depression, the Lower Cretaceous units in the Jiuquan Basin, the Cretaceous-Paleogene units in the Linhe Depression of Inner Mongolia, the Paleogene units in the Jianghan Basin, the Late Cretaceous units in the Songliao Basin, and the Paleogene units in the Bohai BayLiaodong Bay Basin (Figure 1) Previous studies of these strata have proposed specific sedimentary models regarding the factors controlling lacustrine carbonate deposition [2,3,4,5,6]. These studies found that the main controls on reservoir formation include the dominant facies belt, the primary rock (limestone) fabric, dolomitization, and faulting during deep burial [10,11,12,13,14] These studies, primarily focused on deeply-buried Paleozoic marine carbonate strata, whereas shallow-buried Mesozoic or Cenozoic lacustrine carbonate strata have received little attention. The rock structure is relatively simple and is dominated by dolomicrite or microcrystalline dolomite [17]
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