Quantitative evaluation of tight oil reservoirs in the Chang 8 Member of the Yanchang Formation in southern Ordos Basin

Abstract

The precise and quantitative characterization of reservoir properties is the key to efficient development of tight oil reservoirs. In this paper, taking the Chang 8 Member of the Yanchang Formation in the Shuimogou area in the southern Ordos Basin as an example, the sedimentary facies types, microscopic pore structures, diagenesis, influencing factors of physical properties, and hydrocarbon enrichment law of tight oil reservoirs are systematically studied. The research results show that the Chang 8 Member in the study area is a typical delta front subfacies deposit, including distributary channel and inter-distributary bay microfacies. The constructive diagenesis of the Chang 8 Member include dissolution, metasomatism and rupture; while the destructive diagenesis include mechanical compaction and cementation. The Chang 8 reservoir has entered the middle diagenetic stage A. The factors affecting the physical properties of tight oil reservoirs include deposition, compaction, cementation and dissolution. The secondary pores formed by dissolution account for 10–40% of the total surface porosity, with an average value of 24%. Local structures and sediments have significant control over hydrocarbon accumulation. The westward dipping tectonic setting of the northern Shaanxi Slope provides the basic conditions for the migration of oil and gas to the eastern updip areas. The changes of lithology and physical properties in the updip direction of the structure form the blocking conditions for the continued migration of oil and gas, which is conducive to the accumulation of oil and gas. In addition, the main oil reservoirs are mainly distributed along the distributary channel of the delta front, and most of the distributary channel sandstone is more than 20 m thick. The areas of thick sand body with multiple layers in the lateral direction is the main part of oil and gas accumulation. However, the reservoirs formed on the flanks of underwater distributary channels are generally thin and poorly sorted, which is not conducive to the accumulation of hydrocarbons.

Research on pore structure and classification evaluation of tight oil reservoirs based on fractal theory

“…The cast thin section and physical data were analysed, and the rock components were classified into quartz, feldspar, clasts and other colluvium according to the building and destructive effects of mineral types, fillers and colluvium on the reservoir storage properties (Luan et al, 2022). By analysing the cast thin section, it can be seen from the figure below that feldspar is the most abundant in the reservoir, followed by quartz, and the clastic component is dominated by igneous clasts and metamorphic clasts, with some silicate rocks (Baoquan et al, 2021).…”

Formation Characterization and Type Prediction Based on Geophysical Well Log Data in Horizontal Well: A Case Study of Triassic Chang 8 Formation in Shunning Region, Central Ordos Basin

“…Based on the above analysis, we chose two parameters of Rm and fo, which respectively reflect the pore structure and formation quality to establish the formation type indicator parameter. This parameter is calculated as follows: 𝐹𝑇𝐼 = 𝑅 × 𝑓 = 𝑅 × 𝜑 × (1 − 𝑆 ) (17) where FTI is the formation type indicator parameter in the horizontal well. By using Equation ( 17), we process the Chang 8 Formation in 17 horizontal wells, which contain drill stem test (DST) data, in our target region, and FTIs are calculated.…”

“…These mean that current horizontal well classification methods cannot be directly used. In addition, deep research illustrates that local structures and sediments significantly control over hydrocarbon accumulation [17], manifested by differences in pore structure at the microscopic level. Therefore, complicated pore structure is a crucial factor that controls the quality of Chang 8 Member [18].…”

The Micro-Occurrence Mechanisms of Tight Oil: Fluid–Rock Interactions at Microscale Pores, Nanoscale Pores, and Mineral Surfaces

“…In the Ordos Basin, the tight oil resources were preliminarily estimated to be approximately 35.5∼40.6 × 10 8 t [6,7]. The Yanchang Formation, which is characterized by a low porosity, low permeability, and complex pore structure with strong microscopic heterogeneity, serves as a typical tight oil reservoir [8,9].…”

“…Among these, the Chang 6 to Chang 8 Members are considered the most critical tight-oil-producing targets during operations. The proven tight oil reserves of the Chang 6, Chang 7, and Chang 8 Members contain 12.24 × 10 8 , 8.42 × 10 8 , and 9.60 × 10 8 t, respectively [2,7].…”