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Diagenesis significantly influences sandstone reservoir quality, so that a detailed understanding of diagenesis is essential for the identification and prediction of favorable reservoirs. Several studies have been conducted on individual sandstone reservoirs in the Bonan Sag, a typical half-graben, oil-producing sag in eastern China, but few have explicitly considered the potential impact of the asymmetric shape of the sag, which is steep in the north and gentle in the south. This research takes full consideration of the tectonic setting and distribution of source rocks in the sag to compare the style, magnitude and timing of diagenesis in the northern Steep Slope and Sag Zone (SSSZ) and the southern Gentle Slope Zone (GSZ) by means of drill core observations, thin section petrography, high resolution scanning electron microscopy, fluid inclusion microthermometry, as well as cathodoluminescence and carbonate stable isotope analysis. Petrographic observations identify the wide-spread occurrence of authigenic quartz and clay minerals in association with dissolved feldspar, as well as abundant carbonate cements occurring as several distinct generations due to evolving pore fluid composition. The SSSZ has variable carbonate mineralogy while ankerite dominates in the GSZ. The homogenization temperature of fluid inclusions in authigenic quartz in the SSSZ has a wider range (93°C–157.9 °C) and lower starting temperature (93 °C) compared with the GSZ (106.2 °C) and is characterized by more lighter carbonate δ13C and δ18O values (δ18O: −14.8‰~-11.3‰, δ13C: 1.1‰–2.7‰ for the SSSZ; δ18O: −11.9‰~-5.8‰, δ13C: 2.9‰–5.5‰ for the GSZ). The results indicate that (1) Diagenetic conditions in the Bonan Sag evolved from alkaline to acidic to alkaline in general, but with distinct late stage carbonate cementation in SSSZ vs GSZ; (2) the diagenetic processes as well as the nature of pore waters varied across different zones and also temporally. Acidic conditions in the SSSZ started earlier and persisted for longer than in the GSZ. (3) The regional differentiation of the diagenetic processes and pore fluid evolution result from the source rock distribution and burial histories which differ across the Bonan Sag because of the asymmetric shape of the half-graben. We propose a model of fluid evolution over space and time which may provide a new framework for understanding the diagenetic processes and the fluid evolution of oil-producing sags and also contribute to the prediction of favorable reservoirs. • The E s 3 in Bonan Sag went through an alkaline-acid-alkaline diagenetic conditions. • The structure of the sag leads to temporal and spatial differences in diagenesis. • Source rock distribution and burial histories are significant controls in diagenesis. • A temporal and spatial diagenesis evolution model was put forward. [ABSTRACT FROM AUTHOR]

The thermal maturity gradient approach is widely used in comparative studies of mudrocks to assess the evolution of the pore system. The fundamental assumption in this approach is that the compared samples share similar geochemical, mineralogical and textural properties, so that any differences between them can be reasonably attributed to the impact of thermal maturity and burial alteration. However, achieving the desired range of thermal maturity often requires sampling from widely spaced localities which may result in unrecognised microtextural differences owing to regional variations in depositional controls, making it challenging to isolate the effects of thermal alteration on pore system evolution. We use samples from the widely studied siliceous Woodford Shale to highlight the importance of sample screening in characterization studies of pore evolution with progressive thermal maturity. Samples were collected from different cores spanning a gradient of thermal maturity (0.5–1.65 %VRc) across the Cherokee Platform, Arkoma and Anadarko basins, and capturing the wider geographic extent of the formation across Oklahoma. Samples were selected from the same lithofacies type but varying stratigraphic position and screened using the standard bulk geochemical and mineralogical criteria that are widely employed to ensure properties are comparable across a sample set. Notwithstanding this screening step, microscale electron microscope characterization revealed substantial microtextural and compositional differences within the sample set. Our results show that total pore volume is significantly influenced by these seemingly subtle differences, likely because pore evolution pathways vary with the abundance and distribution of primary (i.e. biogenic vs detrital grains) and diagenetic sediment constituents. We conclude that standard sample screening approaches that are limited to bulk mineralogical and geochemical characterization, and cannot identify differences in detrital, biogenic and authigenic components or account for textural differences, are insufficient for thermal gradient studies. A combination of high-resolution imaging, mineral mapping, x-ray diffraction, and low-pressure gas adsorption experiments proved better suited for sample screening ensuring an "apples" to "apples" comparison, as well as for detailed characterization bringing to light the impact of microtextural heterogeneity between samples on measured rock properties. • Sample selection based on thermal maturity and bulk sample analyses alone are poor predictors of microtextural differences controlling pore system evolution. • Alteration of the mineral matrix plays a critical role in the evolution of the pore system in this dataset. • Microtextural differences affect the pathway of pore system evolution and needs consideration during sample screening. • A new generation of automated SEM and mineral mapping facilitated identification of like-to-like samples. [ABSTRACT FROM AUTHOR]