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Abstract
Based on core observations, the microheterogeneity, diagenetic features, diagenetic mineral compositions, and stable isotopes of cements in the calcareous interlayers in the Donghe sandstone were studied by polarizing microscopy, cathodoluminescence microscopy, X-ray diffractometry, isotope ratio mass spectrometry, and other techniques. By determining the proportions of cements of two phases by a statistical method and their clumped isotope values by an end-member method, the multiphase calcareous cementation was shown to be the major contributor to densification. Cluster isotopes revealed that the average formation temperatures of calcareous cements in phases II and III of cementation were 45–50°C and 80–90°C, indicating that they were products of the A and B phases during early diagenesis, respectively. According to the homogenization temperatures of coeval salt-water inclusions associated with hydrocarbon inclusions, which range from 100°C to 130°C, basin modeling revealed that the basin underwent mainly one stage of hydrocarbon charging during 8–5 Ma in the Miocene period. The cements of the two phases in the oil-free calcareous interlayers in the Donghe sandstone, which are the main controlling factor of the oil-water distribution in the reservoir at present, formed much earlier than the oil filling in the oil-bearing sandstone.
Highlights
IntroductionIn the past 30 years, several large oil and gas fields (such as the Donghetang Oilfield, Tazhong 4 Oilfield, and Hadexun Oilfield) have been discovered successively in the Carboniferous Donghe sandstone, which has become an important high-yielding formation in the Tarim Basin [1, 2]
In the past 30 years, several large oil and gas fields have been discovered successively in the Carboniferous Donghe sandstone, which has become an important high-yielding formation in the Tarim Basin [1, 2]
This study is aimed at examining the diagenetic characteristics, mineral composition, and cementation phase of calcareous interlayers in the Donghe sandstone in the Hadexun Oilfield based on microscopic analysis of casting thin sections, cathodoluminescence, and X-ray diffraction (XRD) to clarify the distribution of calcareous cements at the scale of the core and to accurately define the formation time of calcareous cements in interlayers in the Donghe sandstone using clumped isotope analyses and basin modeling to evaluate their influence on hydrocarbon accumulation based on the burial history and thermal history of the basin
Summary
In the past 30 years, several large oil and gas fields (such as the Donghetang Oilfield, Tazhong 4 Oilfield, and Hadexun Oilfield) have been discovered successively in the Carboniferous Donghe sandstone, which has become an important high-yielding formation in the Tarim Basin [1, 2]. The Hadexun Oilfield is the first marine sandstone oilfield with reserves of more than 700 million barrels of oil in place (MBOIP) discovered in the Tarim Basin. Several previous studies have been carried out on the Donghe sandstone, including studies of its sedimentation and provenance, reservoir characteristics, oil and gas source, and accumulation process, and important understandings were achieved [4,5,6,7,8,9,10,11]. The formation and spatial distribution of interlayers in the sandstone are of great significance to the design and adjustment of the development plan of the oil field [12]
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