Characterization of the weathered basement rocks in the Dongping field from the Qaidam Basin, Western China: significance as gas reservoirs

Abstract

Reservoir quality is a critical risk factor in basement reservoirs. Researches into basement reservoirs by petrographic analysis combined with X-ray diffraction, log identification, electron microscopy, field emission scanning electron microscopy, porosity and pulse-decay permeability and core analysis have provided insights into the characterization of the commonality, diversity and difference of the weathered basement rocks as gas reservoirs in the Dongping field. Geological structures, lithology and near-surface processes control the reservoir physical property together. From Wellblock Dp 3 to Wellblock Dp 17, the high uplift gradually transforms into the low slope area towards the center of basin, with the lithology changing as well, which results in different degrees of fracture development in the bedrock in different wellblocks. The basement lithologies are granite, granitic gneiss, and limestone with slate in Wellblock Dp3, Dp1 and Dp17, respectively. Though they all provide effective reservoir space for gas accumulation, the productivity of nature gas shows significant differences. Fractures are the main store space in the three wellblocks. The development of fractures gives rise to secondary porosity around them because of physical weathering and chemical dissolution, but they generate many inhomogeneous fractures and secondary solution pores, whether on the planar distribution or in vertical. In Wellblock Dp3, high angle fractures were generated under the action of structural stress mechanism, with a large number of secondary pores. The porosity is between 0.1 and 23.2%. In Wellblock Dp 1, low angle fractures were the main storage space, with plenty of solution pores mainly in melanocratic minerals. The porosity is between 0.1 and 18.8%. In Wellblock Dp 17, where short and dense fractures developed, the porosity is between 0.1 and 10.3%. The data indicate that the granite in the uplift in Wellblock Dp3 has better reservoir properties due to the stronger physical weathering and chemical dissolution. As the porosity gradually decreases towards the slope and low-lying area, the more favorable exploration area should be the uplift and slope area in the depression area. However, the effective caprock developed locally in Wellblock Dp3, which affected the gas accumulation. Meanwhile, the reservoirs’ petrophysical properties showed distintive variation with different depths in different wellblocks. High productivity layers are under the 200 m, 100 m and 200 m depths from the top of the basement rocks in Wellblock Dp 3, Wellblock Dp 1 and Wellblock Dp 17, respectively. This suggestion in this study will be of significance for guiding oil and gas exploration in front of the Altun Mountains.

Insights into the Weathering Crust Reservoirs of Granitoids: A Case Study from Qinghai Oilfield of Qaidam Basin, Northwest China

“…Meanwhile, other crack fractures that cut through the grains without any filling of authigenic minerals showed a good connectivity, which meant these fractures had a positive effect on the hydrocarbon accumulation (Figure 9h). Brittle minerals are readily cracked under tectonic stress [81]. This type of crack fracture can be extensively observed in quartz, which exists not only in granite and granitic diorite but also in granitic gneiss.…”

The influence of hydrocarbon generation on the sealing capability of mudstone caprock rich in organic matter

“…A full investigation of the effect of OM-derived hydrocarbon generation on the sealing capability of thermally mature mudstone caprock is yet to be conducted. The primary factors influencing the variability in mudstone caprock sealing capability encompass pore structure, pore type, and porosity 22 , 23 . In response to the aforementioned concerns, this paper presents pyrolysis experiments conducted on mudstone caprock rich in OM, with a specific focus on investigating the changes in pore structure, porosity, pore type, and breakthrough pressure at different stages of hydrocarbon generation as thermal maturity increases.…”

Fracture evaluation of the plutonic basement in the Upper Magdalena Basin: Implications for the development of naturally fractured reservoirs in the Northern Andes

“…The distribution of fractures in crystalline rocks is mostly controlled by lithological and structural characteristics, including rock composition, primary textures and structures, fault kinematics and displacement (e.g., Balsamo et al, 2019; Ceccato et al, 2021; Cuong & Warren, 2009; Koning, 2019; McCaffrey et al, 2003, 2020; Nelson, 2001; Rabbel et al, 2021; Torabi & Fossen, 2009; Trice, 2014; Yang et al, 2020). Fault zones can represent both conduits and barriers for the petroleum system; therefore, an appropriate characterization of the structural record and identification of the major controls on the development of fracture networks have been shown to be fundamental for the understanding of petroleum systems in crystalline rocks (Holdsworth et al, 2020; Nanda, 2021; Parnell, 1988; Petford & McCaffrey, 2003; Zou, 2013).…”