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.
Highlights
With the increasing demand for energy, the basement reservoir as a special type of hydrocarbon reservoir has been an important target for exploration all over the world[1,2]
Production of 3.6 × 104 m3 was obtained in the basement rocks in well Dp 3, and multilayered gas reservoirs have been discovered in the Paleogene stratum
The main purpose of this study is to investigate the characteristics of the basement rock reservoir and the effect of gas reserve tests, in addition to the distribution of favorable prospecting layers for the future exploration in the Qaidam Basin
Summary
With the increasing demand for energy, the basement reservoir as a special type of hydrocarbon reservoir has been an important target for exploration all over the world[1,2]. The exploration of basement hydrocarbon reservoirs in Qaidam Basin has continuously exhibited substantial progress with the discovery of basement reservoirs in the Dongping area in front of Altun mountain. Wellblock Dp3 is located in the high fault-uplift zone in front of the Altun Mountains and Well Dp 3 was drilled at a depth of 1850 m 17. The huge thick mudstone deposited in the Jurassic strata are mainly Type II2-III with high content of organic matter of 1.5–3.5% and the vitrinite reflectance is more than 2.0%12,13, from where the generated gas was mainly transported by the deep fractures connecting the Jurassic source rock with the basement rocks in front of the Altun Mountains. Jurassic gas source and anhydrite-bearing caprock are advantageous conditions for the formation of the Dongping field,but the difference of gas yield is decided by basement rock reservoir
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