Conditions Analysis of Continental Shale Gas and Oil Accumulation in Northern and Northeastern China
Jing Bai1,2, Xiongqi Pang1,2, Wenli Jiang3, and Fujie Jiang1,2
1Research Center of Basin and Reservoir, China University of Petroleum, Beijing, China
2State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, China
3Oil & Gas Strategy Research Center, Ministry of Land and Resources, Beijing, China
Different from the marine shale developed in North America, continental shale are widely distributed in China, especially in Eastern and Northeastern China. As different sedimentary environment where shale deposit would contribute to different shale features, intensive and systematic study on the accumulation conditions of continental shale gas and oil is conductive to more reliable evaluation on China’s potential of continental shale gas and oil.
The continental organic-rich shale, widely distributed in Northern and Northeastern China, was formed in semi-deep to deep lakes in Mesozoic and Cenozoic. This paper delves into the distribution features, geochemical and reservoir characteristics, and gas-bearing properties through the single well data, as well as abundant analytic data of shale samples.
The organic rich continental shale, distributed in Northern and Northeastern China, is with large thickness and moderate burial depth. The Triassic Yanchang Formation shale is distributed in central and Southwestern Ordos Basin, with the thickness ranging between 10m~60m and the burial depth between 1750m~2000m; Cretaceous shale mainly develops in Songliao Basin, Northeastern China, and Qingshankou Formation and Nengjiang Formation are the main layers, which are distributed in the north of the basin, with the average thickness over 30m and the burial depth between 1000m and 2000m; the Cenozoic Paleogene shale develops in Bohai Bay Basin, namely Shahejie Formation, and is mainly distributed in depressions. The shale enjoys large thickness, most of which is thicker than 100m, and some even thicker than 300m in certain regions, and its burial depth is relatively deeper, ranging from 1500m to 3000m; the Paleogene Hetaoyuan Formation develops in Naxiang Basin, with the average thickness about 200m and depth between 2000m ~3000m.
The organic abundance of continental shale in Northern and Northeastern China is relatively higher, with the average TOC of each shale layer reaching 2%. The organic abundance of the Triassic Yanchang shale is generally higher than 3%, and for the Cretaceous Qingshankou Formation shale, the average is approximately 2%, with the maximum standing at 8%; the TOC of Paleogene shale ranges from 1% to 5.9%, with the average of 1.55% to 2%. The organic types of the continental shale in Eastern and Northeastern China are mainly of Type I and II. For instance, the Triassic Yanchang Formation shale mainly falls into Type II, while the Cretaceous and Paleogene shale is of Type I and Type II1. The Vitrinite Reflectance results of samples show that the continental shale is at a low evaluation degree, with Ro ranging from 0.4% to 2% and the average about 1.0%, which further demonstrates that the organic matter is in the stage between mature and high mature, in which oil and wet gas are mainly generated.
The main lithology of continental shale reservoirs in Northern and Northeastern China are dark mudstone imbedded with thin sandstone layers. The whole rock analysis of the samples shows that quartz, feldspar, and clay mineral are the dominant mineral composition, and there is a number of brittle mineral, generally speaking, standing at 42% to 60% of the whole. Moreover, the Palaeogene shale has relatively more brittle mineral, which is almost more than 55%; the Cretaceous Nengjiang Formation shale has the most brittle mineral content, reaching 70%. Micropores and microcracks are the major storage space, and the former consists of organic micropores, intergranular pores, intercrystalline pores and dissolved pores. The porosity of the continental shale mainly ranges from 3% to 14%, while the Paleogene shale has relatively higher porosity, most of which runs between 8% and 11%, and the Mesozoic shale of Cretaceous and Triassic is slightly lower, ranging from 5% to 8%.
The continental shale enjoys good gas bearing property. The well testing data shows that industrial oil and gas flows have been obtained from the First Member of Qingshankou Formation and the Second Member of Nengjiang Formation shale in Songliao Basin, as well as Yanchang Formation shale in Ordos Basin and Paleogene Hetaoyun Formation in Naxiang Basin. In addition, the field analytical experiments indicate that the Yanchang Formation shale has the largest gas content, reaching 6.4m3/t, and the isothermal adsorption experiments also show that Yanchang Formation shale has the largest gas content, which is 4.77 m3/t, the followings are Qingshankou Formation and Hetaoyan Formation shale, which are more than 2 m3/t, and the Shahejie Formation has the lowest gas content, ranging from 0.5 m3/t to 1.8 m3/t.
Overall, the continental shale in Northern and Northeastern China has large thickness, wide distribution, mainly distributed in depressions with moderate burial depth, high organic abundance, fine organic types, relatively low material degree, and various pores types, high brittle mineral content which is beneficial to shale gas development, as well as better gas bearing property. The characteristics above mentioned indicate a great potential of shale gas and oil resource in Eastern and Northeastern China, with the Mesozoic Triassic Yanchang Formation shale boasting the most promising prospect.
AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013