Water-Rock Interactions in Two Diagenetic Depth Zones of Offshore Bohai Bay Basin
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2023-12
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Melbourne Authors
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School of Geography, Earth and Atmospheric Sciences
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PhD thesis
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© 2024 Huan Li
Abstract
Bohai Bay Basin is located in eastern China and contains petroliferous fluvial to lacustrine sediments. Two diagenetic depth zones are characterized by elevated total dissolved solid (TDS) concentrations in formation waters in the offshore Bohai Bay Basin, China. This study utilized experimental information, including formation water compositions, petrography, X-ray diffraction clay minerals, crude oil composition, stable isotopes, fluid inclusions, quantitative fault movement analysis, and quantitative grain fluorescence, together with reactive transport simulations to discuss the water-rock interactions occurred in the two zones, the preferential formation of secondary porosity in the middle of a sandstone interval in the deep zone, and the potential connections among shallow, moderate and deep depths through fault movements.
The thesis is composed of six chapters in total. The first chapter provides the background for the analysis of water-rock interactions in siliciclastic sedimentary basins. The second chapter provides the regional geological background of the offshore Bohai Bay Basin. The third chapter discusses the origins of two formation water TDS peaks. The fourth chapter discusses the formation mechanism of higher secondary porosity resulted from feldspar dissolution in the middle of a sandstone interval. The fifth chapter analyses the fault-induced water movement and exchange among different depths and the petrographic results.
The shallow diagenetic zone at a depth of 1,200 – 2,000 m (45 – 70 degree) and the deep diagenetic zone at a depth of 3,000 – 4,000 m (100 – 130 degree) have TDS concentrations exceeding 70,000 mg/L, while the depth interval in between the two zones has TDS concentrations lower than 35,000 mg/L. Processes affecting the deep diagenetic zone were attributed to (1) the dissolution of halite, albite and albitization, which predominantly caused the TDS enrichment, and (2) the dilution of TDS caused by the inter-layer H2O released from the transformation of smectite to illite (salinity reversal at depths > 3,500 m). In comparison, the enrichment in TDS in the shallow diagenetic zone was attributed to hydrocarbon biodegradation, which consumed free water.
A particular formation water component, dissolved organic acids, are enriched in the deep zone, which suggests that they are mainly derived from the thermal decomposition of kerogen in shales. Greater secondary porosity was observed in the central part of sandstones at a depth of 3194–3120 m. This observation differs from previous studies showing secondary porosity mainly in upper and lower parts of sandstones adjacent to shales. An advection model simulating advective transport of low molecular weight organic acids (LMWOA) parallel to the sandstone bedding successfully generated higher secondary porosity in the central part. The central part of the sandstone exhibited better grain sorting (greater depositional porosity) and significantly less early carbonate cements compared to the marginal sandstone parts. Consequently, the central part had greater porosity prior to further mineral dissolution being promoted by LMWOA. The initially higher porosity in the central part allowed for a higher advective flux of LMWOA-rich water and associated lower pH, resulting in decreased oligoclase saturation, higher oligoclase dissolution rates, and ultimately higher secondary porosity.
Abundant secondary porosity was also observed in shallow to intermediate depths (1500–2500 m). However, shallow mudstones are not mature enough and have a limited capacity to generate LMWOA. This study suggests that fault movement activated fluid migration, resulting in the redistribution of organic acids possibly together with CO2 (including naphthenic acid or CO2 from hydrocarbon biodegradation), the acceleration of secondary porosity generation, and the export of by-products. Therefore, under the joint influences of thermal maturation of mudstone and fluid flow induced by fault movement, shallow, intermediate, and deep sandstones exhibit distinctive features of diagenetic fluid-rock reactions. These studies demonstrated the collective controls of geochemical, geophysical, thermal, and biogeochemical processes on the water-rock interactions in the offshore Bohai Bay Basin.
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Water-rock interactions, Reactive transport model, Sandstone diagenesis, Secondary porosity, Quartz cements, Reservoir quality, Fault movement, Hydrocarbon biodegradation, Formation water