Oil & Gas Geology ›› 2019, Vol. 40 ›› Issue (1): 78-91.doi: 10.11743/ogg20190108

• Petroleum Geology • Previous Articles     Next Articles

Tight oil reservoir characteristics of lacustrine mixed marlstone in the Palaeogene Shahejie Formation of Shulu Sag,Bohai Bay Basin

Fu Xiaodong1,2, Wu Jianping3, Shou Jianfeng1,2, Wang Xiaofang1,2, Zhou Jingao1,2, Zhang Tianfu1,2, Guo Yongjun3   

  1. 1. Hangzhou Research Institute of Geology, PetroChina, Hangzhou, Zhejiang 310023, China;
    2. CNPC Key Laboratory of Carbonate Reservoirs, Hangzhou, Zhejiang 310023, China;
    3. North China Oilfield Company, PetroChina, Renqiu, Hebei 062552, China
  • Received:2017-11-21 Revised:2018-10-17 Online:2019-02-28 Published:2018-12-08

Abstract:

Lacustrine marlstones of mixed silisiclastic-carbonate origin are widely developed in the lower third member of the Shahejie Formation(Es3L)in the Shulu Sag,Bohai Bay Basin,and are the main target layers for tight oil exploration.Various data including core analysis,thin section observation,porosity and permeability measurement,mercury intrusion porosimetry,scanning electronic microscope(SEM),CT scanning and nuclear magnetic resonance(NMR) were used to clarify the geologic properties of the special marlstone reservoir of mixed silisiclastic-carbonate origin.Based on the comprehensive analyses in geology and geochemistry,the main controlling factors favorable for reservoirs development were revealed,the criteria for reservoir classification and evaluation were established,and furthermore the favorable plays for tight oil exploration were pointed out.The results indicate that the marlstones in Es3L can be divided into two groups,namely lamellar marlstones of interlaminated mixing and blocky marlstones of structural mixing.The mineral compositions of both types are dominated by calcite,followed by dolomite,and are low in contents of clay,quartz,and feldspar.The re-servoir porosity is typically below 4.0%,and matrix permeability less than 10×10-3 μm2,manifesting its nature of ultra-low porosity and extra-low permeability reservoir.There are diverse types of reservoir pores with complicated structures,which can be divided into two categories and eight subcategories.Most of the matrix pores are nanopores.Reservoir pro-perties and pore structures are distinctly different between marlstones with diverse structures:the pore distribution in the lamellar marlstones is highly heterogeneous,featuring double or multi-porosity media;the lamellar marlstones are better than blocky marlstones in terms of porosity and pore structure.The favorable marlstone reservoirs of mixed silisiclastic-carbonate origin are controlled by sedimentary subfacies,rock fabric,abundance and maturity of organic matters,and density of structural fractures.As for tight oil exploration,the lamellar marlstones with high TOC that are deposited in inner slope and semi-deep and deep lacustrine subfacies of trough areas are the favorable targets for tight oil exploration.

Key words: mixed marlstone, tight oil reservoir, Shahejie Formation, Shulu Sag, Bohai Bay Basin

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