Crude oil geochemical features, charging model and exploration practice in slope zone of the Oriente Basin

doi:10.13745/j.esf.sf.2020.6.38

Abstract

Abstract:

The slope zone of the Oriente Basin has a complex crude oil distribution according to °API. Heavy oil extraction in the basin is difficult and costly, accompanied by unclear distribution of medium and light crude oil, which greatly hinders oil and gas exploration in this area. In this paper, starting with geochemical analysis of crude oil, we clarified the formation mechanism of heavy oil and established a charging model for crude oil by analyzing its distribution and geochemical characteristics. We also identified the favorable prospecting areas for medium and light crude oil to provide effective guidance for oil and gas exploration. The results show: (1) Crude oil samples from the slope zone had a front peak in their n-Alkane distribution, indicating marine source. (2) The group composition of crude oil displayed a “double low+double high” feature, suggesting the crude oil comes from the low maturity-maturity stage of source rocks. (3) Biodegradation is the major reason for heavy oil formation, evidenced by (i) the well developed UCM phenomenon in the total carbon ion flow chart; and (ii) the detection of 25-norhopane marker compounds. (4) Degraded and normal crude oil co-exist in the same oil sample according to geochemical analysis, indicating crude oil is the results of mixing two stages of oil charging. Based on the results, a two-stage oil filling model is established: After early oil charging, crude oil is generally degraded and becomes heavier in the mid-stage; then after late-stage oil charging, the mixing of later stage normal oil and degraded crude oil controls the planar distribution of crude oil. The area in the down dip direction of shale belt in the slope zone is the favorable area for normal oil exploration as proved by successful drilling of exploration wells.

Key words: geochemical feature, heavy oil formation mechanism, biodegradation, mixed oil charging

CLC Number:

P618.130.2

ZHANG Zhiwei, MA Zhongzhen, ZHOU Yubing, YANG Xiaofa, WANG Dandan, LIU Yaming, ZHAO Yongbin. Crude oil geochemical features, charging model and exploration practice in slope zone of the Oriente Basin[J]. Earth Science Frontiers, 2021, 28(4): 316-326.

Figures/Tables 10

Fig.1 Distribution of sampling sites in the Oriente Basin

Fig.2 Vertical distribution features of crude oil API in T block

Fig.3 Layout map of oil reservoirs in the Oriente Basin. Modified after [6].

Fig.4 Distribution of oil reservoir and crude oil API in M1ss section of T block

Fig.5 Total ion chromatograms (TIC) of saturated hydrocarbon for crude oil samples from T block

Fig.6 GC/MS spectra of crude oil from Well C7 identifying 25-norhopane biomarker compound

Fig.7 Thermal evolution history and organic matters maturity for Well Piuntza-1X

Fig.8 Photomicrographs of different types of fluid inclusions in sandstone samples from slope belt under different lights

Fig.9 Distribution of homogenization temperature for fluid inclusions

Fig.10 Hydrocarbon charging model for the Oriente Basin slope belt

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