Abstract
In the Ordos Basin, the Chang 8 oil reservoir of the Triassic Yanchang Formation is the main target of oil exploration and development and there are many factors affecting the hydrocarbon accumulation. Well G20 is an important exploration well in the Yanchi area in the northwestern part of the basin, and the core of the Chang 8 reservoir was sampled and tested to determine the source rocks, trace element composition, mineral composition, reservoir physical properties, and oil-bearing properties. The results show that the rock retrieved exhibits delta plain subfacies; the range of the Chang 8 water body was large, with a gentle slope, and the climate was relatively dry. These findings suggest that this reservoir was deposited in an environment in which sedimentary sand body could easily form. The Chang 8 sandstone reservoir pores are dominated by intergranular pores and feldspar intragranular dissolution pores, indicating that the Chang 8 reservoir is a low-porosity and low-permeability reservoir. Chang 7 source rocks from this area have a type I-II1 hydrocarbon-generating potential, with an average total organic carbon (TOC) content of 5.99% and vitrinite reflectance (Ro) value of 0.48%. Combined with the regional sedimentary evolution, tectonic movement, and reservoir distribution, it is considered that due to the lack of lithologic traps or low-amplitude structural traps, G20 produced water in well testing. The Chang 8 oil reservoir in the Yanchi area can be divided into structural reservoirs and structural-lithologic reservoirs. The conventional oil and gas reservoir exploration ideas of “thick sand belt” and “reservoir sweet spot” are not applicable here. The lithology traps or low-amplitude structural traps and areas with good preservation conditions are the main directions for the next phase of exploration in the northwestern part of the Ordos Basin.
Highlights
Introduction e Ordos Basin is the second-largest sedimentary basin in China. e Triassic Yanchang Formation in the Ordos Basin is mainly composed of a set of inland clastic sedimentary layers with thicknesses of 500–1200 m. e Yanchang Formation can be divided into 10 oil-bearing members (Chang 10 to Chang 1, from the bottom to the top) and is the main oil-producing layer in the basin. e Chang 7 oil-bearing member is mainly composed of a set of gray-black mudstones with a high organic carbon content and an organic matter type that is favorable for oil generation
Sedimentary Characteristics. e five signs used to judge the sedimentary environment of well G20 are as follows: (1) the primary color characteristics of the sediments, which can indicate the sedimentary environment; (2) the changes in the vertical grain size and sedimentary structure; (3) the presence of plant stalks, which can indicate the underwater environment near the lake shoreline; (4) the presence of coal seams, which can indicate the onshore sedimentary environment; and (5) the characteristics of the logging facies [20]
E Chang 8 coring section (1650.1–1688.2 m) of well G20 is mainly composed of grayish-white thick-bedded fine siltstone, with a small amount of argillaceous siltstone and silty mudstone, and local deposits of argillaceous lamina and banded oil. is section is mainly composed of distributary channel and distributary bay deposits. e erosion surface at the bottom of the sand body is common, and the upward transition shows thin interbedding of siltstone or silty fine sandstone, siltstone, and argillaceous siltstone. e grain size fines upward. e primary colors of the interbedded mudstone are light gray to black, indicating that the formation and oxidation-reduction interface were above the surface of water
Summary
Taking well G20 as an example, the study focuses on the observation of the sedimentary structure phenomenon of Chang 8, and the total length of the core is 38.1 m. To determine the rock type, reservoir physical properties, and oilbearing properties, 255 core samples were collected. 187 samples were analyzed for porosity, permeability, and detrital composition; 11 samples were analyzed by fluorescence and nuclear magnetic resonance; 13 samples were analyzed by scanning electron microscopy; 15 samples were analyzed by cathodoluminescence; and 3 oil sand samples were analyzed by saturated hydrocarbon mass spectrometry. Gas chromatography-mass spectrometry (GC-MS) analysis was carried out using ermo-Fisher Trace-DSQ II, which can detect the combination characteristics of biomarkers in crude oil, saturated hydrocarbon, aromatic hydrocarbon, and other components. E rock type and composition of the sample were determined by polarizing microscopy with a LEICA 4500P apparatus. All the core sample collection and observation work was carried out in the CNPC Changqing Oilfield
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