关键词: 洞庭盆地, 生物气, 广域电磁法, 汞气测量, 地球化学异常, 第四系, 游离烃, 酸解烃

Abstract: The Dongting Basin, located at the edge of the Paleozoic in Xiangxi, experienced two stages of tectonic cycles involving mainly the Mesozoic Late Cretaceous basin and the Cenozoic fault block. This rift basin is a result of the Yanshan (Mesozoic) and Himalayan (Cenozoic) movements. There are persistent sand and shale deposits on the Cretaceous in the basin, making the sedimentary facies of the Quaternary rivers and lakes geologically favorable for biogas generation. Based on the 20132015 geochemical exploration of the shallow sediments in the Dongting Basin, we have a clear understanding of the distribution of methane gas anomalies in the basin. The geochemical anomalies indicated that the Dongting Basin biogas resources, on the northern slopes of the Yuanjiang depression in the northern Dongting Basin, QingshuzuiHebazhen area, present the most likely prospective breakthroughs. In order to trace the source of methane gas leakage, and advance our understanding of geochemical anomaly and shallow biogas system research, we completed five widearea electromagnetic sounding and two mercury gas measurement profiles in the biogas prospective area discovered in 20152016. This accomplishment was aimed to reveal the Quaternary tectonic distribution and geochemical anomaly mechanism through the comparative study of integrated geophysical and geochemical profiles. It provided a scientific basis for the Dongting Basin biogas resource prospecting and target prediction as well as guidance for the selection of Quaternary gas prospecting methodology. The results showed: (1) The Quaternary system in Dongting area can satisfy the “sourcereservoircap” condition of biogas system and has biogas resource potential; (2) The sediment free methane anomaly is caused by the Quaternary gas accumulation along the fault zone or by leakage due to the sediment layer movement towards earth surface; (3) The area encompassing the northwestern Dongting Basin, southern P2 section, and the intersection of northern P2 line and P3/P4 sections, have good “selfgeneration and selfstorage” capability, and are the most promising targets in the prospective area; (4) The Sijihong area, located in the eastern prospective area of northern Dongting Lake, along the southern P6 line and near the Yuanjiang depression center, possesses favorable conditions to form the socalled “selfgeneration and selfstorage” and “lowergeneration and upperstorage” biogas deposits; (5) Shallow sedimentary geochemical exploration is conducive to the discovery of gas leakage associated with biogas, and it is also useful in evaluating underlying depositional system, reservoir development and characterization, and targeting geophysical exploration areas. The combination of geochemistry and widearea electromagnetic profile measurements provides a fast and economical means of biogas prospecting.

Key words: Dongting Basin, biogas, wide-field electromagnetic method, mercury gas measurement, geochemical anomaly, Quaternary, free hydrocarbon, acid hydrolysis