--> --> Abstract: Origin and Reservoir Forming Mechanism of Mixed Sediments of the Inland Saline Lacustrine Basin on Plateau– Case Study on the Neogene in the Northwest Qaidam Basin, by Wei Xu, Kaiyuan Chen, and Zhengyu Li; #90180 (2013)

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Origin and Reservoir Forming Mechanism of Mixed Sediments of the Inland Saline Lacustrine Basin on Plateau– Case Study on the Neogene in the Northwest Qaidam Basin

Wei Xu, Kaiyuan Chen, and Zhengyu Li
School of Energy, China University of Geosciences, Beijing, China

The Qindam basin was a typical inland saline lacustrine basin on plateau during the Tertiary period. Because of the arid climate and the lack of provenance, the sediments had the characteristic of fine-grained. The grain-size analysis in laboratory revealed that the median size of the sediment is between 12 and 43μm. It dominated the feature of poor physical property with lower porosity and permeability of the reservoir.

The saline lacustrine fine-grained deposits in the Neogene of the Qindam basin have the typical characteristics of mixed sediments, which are composed of mixtures of siliciclastic and carbonate material. Mount(1984) explicitly put forward the concept of mixed sediments for the first time. By definition, the mixed deposits mean that terrigenous clastics and carbonates are mixed within the same bed, which identified as the structural mixed deposits. Broadly speaking, the mixed deposits also include the interbedding of terrigenous clastics and carbonates, which identified as the interbedded and intercalated mixed deposits. He also grouped the processes responsible for the mixing into four categories: (1) punctuated mixing; (2)facies mixing; (3)in situ mixing; (4)source mixing.

Mounting studies have been carried out on the mixed deposits reservoir of the northwest Qaidam basin. The mixed sediments reservoirs in Nanyishan of Qaidam basin were divided into three types of deformed bedding mixed sediments layer, horizontal bedding mixed sediments layer and massive bedding mixed sediments layer according to bedding structures of rock layers in the reservoirs. The analysis on the origin, composition, rock type, pore structure and physical properties of the rock layers show that the three kinds of mixed sediments layers correspond to three types of sedimentary environments (Zhang, et al, 2006). The features of lithology, reservoir space, physical property, and diagenesis of the middle-deep mixed deposits reservoir in the Nanyishan area of northwestern Qaidam basin were elaborated systematically, and the reservoir forming mechanism was further addressed based on the features. (Feng, et al, 2010). Most of studies mainly focused on various characteristics of the mixed deposits reservoir, but few proposed a genetic classification of the mixed deposits of the northwest Qaidam basin.

Current study innovatively identifies two sedimentary processes that lead to the formation of mixed sediments in the northwest Qaidam basin. The features of petrology, reservoir space, physical property, and diagenesis of the Tertiary mixed deposits reservoir in the northwest Qaidam basin were studied systematically. The sedimentary model of the mixed deposits and the reservoir forming mechanism was further discussed based on the features. The conclusions presented here are drawn from relatively comprehensive observations of more than 1,000 conventional thin-sections, casting thin-sections and scanning electronic microscope images, and from analysis of cores, whole rock mineral, electronic probe and physical property.

In the northwest Qaidam basin the formation of sediments composed of mixtures of siliciclastic and carbonate material involves a variety of biologic and sedimentologic processes. These processes can be grouped into two general categories: (1) facies mixing, (2) algae mixing. The facies mixing can be further divided into two subcategories: (1) interstratified mixing, (2) structural mixing (Figure 1), likewise, the algae mixing can be further divided into two subcategories: (1) algae bonding mixing, (2) slumping mixing. It should be noted that the slumping mixing occurs in the sandy algal limestone stratum. The above four types of mixing process indicate different sedimentary environments and hydrodynamic conditions. It suggests a sedimentary model of the mixed deposits in the northwest Qaidam basin based on statistical and sedimentologic analysis.

However, the model is an initial investigation which warrants further testing and refinement as more data become available.

Four main pore types are identified in the facies mixing sediments reservoir, which are (1) intergranular dissolution pores in the siliciclastic beds, (2) dissolution pores in the carbonate material, (3) intercrystal micropores in the carbonate material, (4) microfractures. The reservoir space types of algae mixing sediments reservoir are relative simple and predominant with dissolution pores, which contains moldic pores and dissolution pores in the carbonate material.

Based on the observation of large amounts of microscopic data, this study concludes that the diagenesis of mixed sediments reservoir formation in the Neogene of the Qindam basin includes compaction, cementation, dissolution and the process of fracturing.

According to above results, the forming mechanism of the facies mixing sediment reservoir includes sedimentary environment, organic acid dissolution and process of fracturing. For the algae mixing sediments reservoir, it is dominated by sedimentary environment, hydrodynamic conditions, organic acid dissolution and atmospheric water leaching along the fault zone.

In conclusion, current study provides valuable information for both basic reseach and petroleum exploration and exploitation of saline lacustrine mixed sediments reservoir.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013