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Characterizing Hydrocarbon Source under High Maturation by Rare Earth Element Geochemistry: The Neoproterozoic Dengying Formation of the Sichuan Basin, Southwest China

Jian Cao¹, Chunhua Shi¹, Xiucheng Tan², Wei Zeng², Bing Luo³, and Yu Yang³
¹Department of Earth Sciences, Nanjing University, Jiangsu, China
²School of Resource and Environment, Southwest Petroleum University, Chengdu, Sichuan, China
³RIPED, PetroChina Southwest Oil and Gas Field Company, Chengdu, Sichuan, China

The hydrocarbon for exploration under high maturation is often gas. The gas, as it is universally acknowledged, has extremely limited geochemical data in comparison with oil, e.g., chemical components and isotopic compositions. As a consequence, the hydrocarbon (gas in particular) source is remarkably hard to characterize. This is one of the key issues in the study of petroleum geology and geochemistry. Here we present a new attempt of rare earth element (REE) geochemical constraints using the Neoproterozoic (Sinian) Dengying Formation of the Sichuan Basin, southwest China as a case study, which has obtained great exploration success recently and is a research highlight in China.

Natural gas in the Dengying Formation is the product of oil cracking. This indicates that the gas source can be constrained by analyzing reservoir bitumen, which is the other end-member product of the oil-cracking process. According to this, we analyzed the developing features of source rocks at first step. Then, the representative REE parameters that can distinguish these source rocks were studied. The parameters were compared between source rocks and bitumens, providing the constraints on gas and bitumen source.

According to source-rock developing features, the potential source rocks of the Dengying gas can be divided into two categories and five sets. Two categories refer to the mudstone and carbonate rocks in lithology. Five sets refer to the source rocks in age, including the mudstone of the Neoproterozoic Doushantuo Formation, the mudstone of the third member of the Dengying Formation, the muddy dolomite of the early Cambrian Maidiping Formation and the mudstone of the early Cambrian Qiongzhusi Formation. In addition, the algal dolomite of the second and fourth members of the Dengying Formation is another possibility. Organic petrological observation shows that the organic precursor of the mudstones is composed mainly of bacteria and lower alga with some likely contribution from macro benthic alga, which has not been found in carbonate source rocks. Organic geochemistry of the source rocks reveals that they vary in organic matter abundance. It is shown that: (1) high-quality source rocks with TOC>1.0% are developed in each set of these source rocks; (2) the mudstones are generally better than the carbonate rocks; (3) the Qiongzhusi and Doushantuo formations are better than the others; and (4) the rocks in the southwestern areas are of relatively low quality.

We attempt to distinguish the five sets of source rocks based on Q-type cluster analysis of trace and REE elements. It is shown that the REEs have good response to source rocks except the Dengying algal dolomites. The results are in good consistency with geological setting, reflecting the good application of the REEs. In contrast, trace elements have fairly poor response to the source rocks, indicating that the trace elements have been changed to certain degrees in the source-rock evolution and are not stable as the REEs. As a result, they are not good at the distinguishing of the source rocks.

The five sets of source rocks were further normalized by post-Archean Australian shale (PAAS), also for the distinguishing of them. Similar results with the above Q-type cluster analysis were obtained, i.e., REEs are better than the trace elements in the distinguishing. As shown in Fig. 1, for the PAAS-normalized pattern of the five potential source rocks, (1) the Qiongzhusi mudstone is generally flat with slightly convex in light REEs; (2) the Maidiping muddy dolomite lean to the left with remarkable negative Ce anomaly and positive Y anomaly; (3) the mudstone of the third member of the Dengying Formation is also generally flat like the Qiongzhusi mudstone and has fairly hat-type feature, remarkable negative Ce anomaly and slightly positive Y anomaly; (4) the Dengying dolomite is characterized by hat-type, negative Ce anomaly, positive Eu anomaly and positive Y anomaly; and (5) the Doushantuo mudstone is generally flat with slightly light REEs depletion and negative Ce anomaly. These differences provide good foundation for the hydrocarbon source correlation.

Eleven REE parameters that can indicate basic features of source rocks (e.g., depositional environment) were analyzed for obtaining indicative parameters of hydrocarbon source correlation, including REE+Y, Y/Ho, LREE/HREE, Ce/Y, (La/Sm)PAAS, (Gd/Lu)PAAS, Eu/Eu*, Ce/Ce*, Y/Y*, La/La* and Gd/Gd*. It is shown that 6 of these 11 parameters are relatively effective, including Y/Ho, LREE/HREE, Ce/Y, Ce/Ce*, Y/Y* and La/La*. They are characterized mainly by two features. First, the parameters vary slightly in each set of source rock, indicating they are stable during the organic evolution. Second, the values vary largely between different sets of source rocks, thereby favorable for distinguishing the rocks.

Based on the above results, we propose the two steps in constraining hydrocarbon source of the Dengying gas and bitumen. The first step is to qualitatively ascertain the source by Q-type cluster analysis and normalization patterns, while the second step is to conduct a quantitative calculation by indicative parameters. According to this two-step method, we investigate the hydrocarbon source of the Dengying gas and bitumen. Results show that the gas and bitumen in the Ziyang and Weiyuan areas are derived mainly from the Qiongzhusi and Doushantuo mudstones, whose contribution is approximately 94% and 6% and 81% and 19% for the Ziyang and Weiyuan areas, respectively. In the southwestern basin margin area, the gas/bitumen was sourced mainly from the Maidiping muddy dolomite at approximately 47% and Dengying algal dolomite at approximately 53%. In contrast, the central basin area is the most complex with multiple potential sources from the mudstones of the Qiongzhusi Formation (approximately 33%), third member of the Dengying Formation (approximately 17%) and the Doushantuo Formation (approximately 33%) and the Dengying algal dolomite (approximately 17%).

These results, in comparison with previous works, are more focused and in better consistency with geological setting. In particular, we provide quantitative calculation for the first time. The REE geochemical method is a relatively new attempt, which has not been reported previously. Therefore, the method and the 6 parameters also have general implications for studying hydrocarbon source under high maturation.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013