151 / 2022-02-27 14:52:17

海拉尔盆地南屯组湖相碳酸盐岩的特征及意义

lacustrine carbonate,Nantun formation,Hailar Basin,geological implication

Cretaceous strata of Hailar Basin is a typical sediments of continental lacustrine rift basin, which has nearly 40 years of exploration and development history. In previous studies, there are few reports of lacustrine carbonate rocks in the basin. After re-examination of old wells, two types of lacustrine carbonate rocks, micrite dolomite and marlstone, were recently observed in the top cores of the first member of the Nantun Formation in multiple wells in different depression zones in the basin. The thickness of the lacustrine carbonate are about 0.2 m, and appeared in rhythm with dark gray mudstone and siltstone.To study the geological implications of these lacustrine carbonate sedimentary faces, sequence evolution and paleoenvironment, this paper were analyzed comprehensively based on core and thin section analysis and the application of techniques such as Integrated Prediction Error Filter Analysis(INPEFA) and trace elemental geochemistry tests. Three basic understandings were obtained.

 Firstly, lacustrine carbonate can indicate semi-deep to deep lake environment. These two types of lacustrine carbonate rocks are characterized by thin thickness, and accompanied with mudstone. The dolomite has a laminated micrite structure, with less terrigenous clastic sediments and no bioclastic structure in composition. It shows a deep lacustrine facies. But in marlstone the content of terrigenous clastic sediments is about 1% to 10%, with a gradually increasing trend from bottom to top. In thin sections, the degree of quartz particle rounding are angular to sub-angular. It shows a semi-deep lacustrine facies with a gradually increasing supply.

Secondly, lacustrine carbonate can indicate maximum flooding surface. With the analysis of INPEFA, it can be identified that Nantun Formation has a whole three-order sequence with transgress during N1III~N1I and regress after N10 period. Marlstone and micrite dolomite developed before and after the occurrence of the largest lake flooding, respectively,indicating the beginning and end of the condensed section.

Thirdly, dolomite section reflects extreme paleoclimate and paleowater conditions. Sr/Cu value and V/(V+Ni) value can reflect paleoclimate and redox conditions of sedimentary water. Especially small rifted lake basin like Hailar, the content of Sr and Cu in the water body is more sensitive to the climate response. The dolomite section in Hailar has the characteristics of high Sr/Cu value, medium Rb/K value and medium V/(V+Ni) value. It indicates the extremely dry and heat paleoclimate and saline and anaerobic paleowater condition.

When compared with the lacustrine carbonate rocks which have good oil and gas shows like Nenjiang Formation in Songliao Basin, the Tengger Formation in Erlian Basin, the Shangganchaigou Formation in Qaidam Bain, the Shahejie Formation in Bohai Bay Basin, the carbonate rocks currently discovered in Hailar Basin shows a different characteristic. They are characterized by thin thickness, fine structure, and associated mudstone. Although large-scale reservoirs haven’t been found in these type of carbonate yet, as a special environment indicator in lacustrine sediment system they have high value of study.