Element, Isotope and Fluid Inclusion Geochemistry of Reservoir Dolomite in the Neoproterozoic Dengying Formation of the Sichuan Basin, Southwest China

Chunhua Shi¹, Jian Cao¹, 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

Recently, the Neoproterozoic (Sinian) Dengying Formation in the Sichuan Basin, southwest China obtained great exploration success, with Well Gaoshi 1 produced gas at approximately 1.02×10⁶ cubic meters per day. To provide new data on the exploration, we investigate fluid geochemistry of the reservoir dolomite here on the base of petrographic observations, mainly including element, isotope and fluid inclusion characteristics.

The Dengying dolomites can be distinguished into two categories according to their solution features (Fig. 1). Of the two categories, the fluid geochemistry of the dolomites with little solution mainly indicates the dolomitization fluid and their origin. In contrast, the fluid geochemistry of the dolomites with solution mainly indicates the nature of the solution fluid and origin of the reservoir. These are the two main specific aims of this study.

The dolomites with little solution can be further divided into microcrystalline, powder, fine-grained dolomites and doloarenites. In contrast, the dolomites with solution can be further divided into normal solution and re-crystallined dolomite and a somewhat special type with grape-like shapes. These multiple lithologic types imply the complex fluid activating features of the dolomites.

Trace and rare earth element geochemical analysis shows that for the dolomites with little solution the content of Sr, Mn and Fe varies between 38.1-89.0 ppm, 153-860 ppm and 1181-2179 ppm, respectively. The ratio of Mn/Sr is between 4.0-18.7. In REE normalized pattern, they show hat-type characteristics to varying degrees. In typical REE parameters, Ce/Ce*=0.36-0.87, Y/Y*=1.36-1.76, La/La*=1.04-1.72, Gd/Gd*=0.94-1.20, Y/Ho=38.0-49.4 and Eu/Eu*=0.54-2.02. According to these results, we infer that the dolomitization fluid is mainly seawater, typically evidenced by negative Ce anomaly, positive Y anomaly, Y/Ho values and Sr abundance. In addition, there is some hydrothermal influence based on positive Eu anomaly, and some meteoric influence based on typical element abundance including Sr, Mn and Fe. The hat-type characteristics of REE indicate mixing of multiple types of fluids. In summary, the dolomitization fluid is derived mainly from seawater; there is also some contribution from hydrothermal and meteoric waters, whose influence is not common and varies among samples.

For the dolomites with solution, the content of Sr, Mn and Fe varies between 39.1-71.4 ppm, 100-857 ppm and 1054-2238 ppm, respectively. The ratio of Mn/Sr is between 1.9-12.0. In the REE normalized pattern, they show hat-type and flat characteristics to varying degrees. In typical REE parameters, Ce/Ce*=0.53-0.87, Y/Y*=1.37-2.10, La/La*=1.06-1.49, Gd/Gd*=0.93-1.21, Y/Ho=39.0-58.9 and Eu/Eu*=1.21-2.11. These results reflect differences from the dolomites with little solution, indicative of more severe influences from hydrothermal water, meteoric water and petroleum-bearing fluid. Thus, the solution fluid may have multiple types of sources, as typically evidenced by the grape-shaped dolomite.

Strontium isotope (⁸⁷Sr/⁸⁶Sr) of the dolomites with little solution varies between 0.7087 and 0.7093 except one relatively high value being 0.7113. These results are approximately in the isotope range of seawater in the Dengying period (0.7080-0.7090). Thus, it can be indicated that the dolomitization fluid is mainly seawater, with some input from crust, which is typified by relatively high isotopic values. This may be caused by the material carrying from meteoric and hydrothermal waters. This understanding is generally consistent with the above results obtained from element geochemistry. In contrast, the ⁸⁷Sr/⁸⁶Sr values of the dolomites with solution are between 0.7090 and 0.7108, remarkably higher than those of the dolomites with little solution. This implies that the solution fluid has sources from meteoric and hydrothermal waters, also consistent with the above results obtained from element geochemistry.

Carbon and oxygen isotopes of the dolomites generally vary between -5.51and 3.59‰ and -12.72 and -3.33‰, respectively. For the dolomites with little solution, they are -5.51‰ and 2.99‰ (average at 0.91‰) and -12.72‰ and -3.33‰ (average at -6.31‰), respectively, being relatively lower than typical dolomites with marine origin. Thus, it may be indicated that the dolomitization is somewhat influenced by hydrothermal and meteoric waters to varying degrees. In contrast, the carbon and oxygen isotopes of the dolomites with solution range between 1.65‰ and 3.59‰ (average at 2.71‰) and -7.55‰ and -3.96‰ (average at -5.78‰), respectively. This implies that the oxygen isotope is better in response to the solution compared with the carbon isotopes. In addition, the bituminous dolomites have carbon and oxygen isotopes between -1.21‰ and 1.8‰ (average at 0.67‰) and -7.08‰ and -3.38‰ (average at -5.54‰), respectively, indicating the influence from petroleum-bearing fluid.

Fluid inclusion analysis of dolomites with solution shows that the inclusion homogenization temperature has three ranges in general, i.e., 60 , 90-116  and 120-190 . They roughly correspond to the three stages of fluid evolution of organic matter. Thus, it may be indicated that the solution fluid has relation to organic matter evolution.

Integrating the above results and discussion, we propose that the dolomites are not primary in origin as previously suggested and the dolomitization fluid is derived mainly from seawater with some contribution from hydrothermal and meteoric waters to varying degrees. This clarifies previous understandings. It has been suggested that there are hydrothermal dolomites. In the study of reservoir solution and origin, supergenetic solution has been believed to be the main controlling point. Through our work, we propose that burial solution also has important impacts and the solution fluid has complex sources, which may be related to hydrothermal water and the evolution of petroleum-bearing fluid.

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