Earth Science Frontiers ›› 2023, Vol. 30 ›› Issue (2): 122-138.DOI: 10.13745/j.esf.sf.2022.2.61

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Sequence stratigraphic framework and sedimentary evolution of the Cretaceous in southern Iraq

HAN Haiying1(), GUO Rui1,*(), WANG Jun1, QIN Guosheng1, SUN Xiaowei2, YU Yichang3, SU Haiyang1, GAO Yang4   

  1. 1. Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China
    2. PetroChina Hangzhou Research Institute of Geology, Hangzhou 310023, China
    3. China National Oil and Gas Exploration and Development Company Ltd., Beijing 100034, China
    4. Strategic Research Center of Oil and Gas Resources, Ministry of Natural Resources, Beijing 100034, China
  • Received:2021-05-10 Revised:2021-12-10 Online:2023-03-25 Published:2023-01-05
  • Contact: GUO Rui

Abstract:

The Cretaceous is an important exploration and development target in southern Iraq. However, a lack of systematic sequence stratigraphic study among major oilfields has greatly restricted fine planning for the oilfield development and production. Therefore, it has great theoretical and practical significance to investigate the sequence stratigraphy and sedimentary systems in the study area. Here, guided by sequence stratigraphy and sedimentology theory, and based on the core data, mud logs, well logs and seismic data, the sequence boundaries are identified and sequences are delineated; meanwhile the evolutionary characteristics of the sedimentary system are analyzed. According to the results, the Cretaceous in southern Iraq is mainly composed of skeletal limestone, and the sequence interface types include unconformity, exposed surface, lithology abrupt surface and sedimentary facies transition surface types. The Cretaceous is divided into 2 first-level sequences (AP8, AP9), 6 second-level sequences (SS1-SS6) and 22 third-level sequences (SQ1-SQ22); the duration of each 3rd sequence varies from 2-4 Ma. In the Lower Cretaceous, SS1 develops, from south to north, ramp-to-basin sedimentary system, and sediments evolve upward from basinal mudstone to inner ramp grainstone; sediments of the lower SS2 gradually evolve, SW-NE, from delta sandstone to deep-water shelf carbonates, while sediments of the upper part evolve from basinal mudstone to shallow-water shelf carbonates; and in SS3, sediments of the lower part evolve, SW-NE, from braided river coarse sandstone to shallow sea fine sandstone, while marine carbonate rock develops in the upper part. In the Upper Cretaceous, SS4 develops, from west to east, upward shallowing depositional successions of deep shelf-shallow water carbonate ramp-platform sedimentary systems; while fine-grained carbonate ramp sedimentary system, dominated by planktonic foraminifera limestone, develops in SS5; and in SS6, shallowing upward carbonate ramp sedimentary system develops in three stages, and shelf dolomite develops on the uppermost part. Carbonate ramp shoals, platform marginal shoals and two-stage sandstone deposits are the most favorable exploration and development targets in this area. The tectonic movement plays an important role in controlling the development and distribution of the inner ramps and marginal reefs and shoals.

Key words: Cretaceous, Iraq, sequence stratigraphy, sedimentary evolution, skeletal limestone

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