--> Diagenetic Evolution and Mass Transfer of Deep-Water Mudstones in the Middle of the Third Member of the Shahejie Formation in the Dongying Sag, Jiyang Depression, Eastern China

AAPG ACE 2018

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Diagenetic Evolution and Mass Transfer of Deep-Water Mudstones in the Middle of the Third Member of the Shahejie Formation in the Dongying Sag, Jiyang Depression, Eastern China

Abstract

The diagenetic evolution and mass transfer of mudstones have a significant influence on both mudstone and sandstone reservoir quality. A variety of methods were applied to investigate the diagenetic evolution and mass transfer of the mudstones in the middle of the third member of the Shahejie Formation (Es3z) which was formed in a lacustrine basin. Optical microscopy work suggests the mudstones are dominated by clay minerals (56.8%) followed by carbonate (18.2 %), quartz (14.9 %), feldspar (8.4 %), pyrite (0.9 %), anhydrite (0.8 %), and total organic carbon (TOC: 2.4 %). Cementation and replacement are the main diagenetic events in the mudstone of Es3z. Carbonates and clay minerals are the most common cements, but occasionally authigenic pyrite, albite, and quartz are also present. In the eogenetic stage, the diagenetic events are represented by precipitation of pyrite, siderite, dolomite, and calcite along with the transformation of K-feldspar into kaolinite. When transited to the mesogenetic stage, the main diagenetic events are dominated by precipitation of fracture-filling calcite, ferroan calcite, ankerite, microcrystalline authigenic quartz, quartz overgrowth, and transformation of K-feldspar into illite and albite. The small variation of the Nb/Ta ratio and Zr/Hf ratio of all the mudstone samples, as well as the almost identical distribution pattern of the REE and trace elements, indicates common felsic igneous rocks provenance. The CIA values of the mudstones in the Es3z vary from 50.59 to 65.05, the PIA values vary from 50.86 to 75.58 confirm a low-intensity weathering of the source materials. Thus, the variation in the bulk-rock geochemical composition of mudstone may due to mass transfer with the interbedded sandstone during diagenetic processes. The changes ratio of aim elements to immobile elements with burial depth and mass balance calculation indicated that aluminum and silicon were retained by the precipitation of authigenic minerals in the mudstone. Potassium was transferred from sandstone to mudstone, while the versa for calcium. Sandstone in Es3z as a reservoir has its quality been affected mainly by the carbonate content in the mudstones.