Abstract: Constraining basin thermal history is a key part of reservoir characterization in carbonate rocks. Conventional palaeothermometric approaches cannot always be used: fluid inclusions may be reset or not present, while δ 18 O palaeothermometry requires an assumption on the parent fluid composition. The clumped isotope palaeothermometer, however, is a promising technique for constraining the thermal history of basins. In this study, we test if clumped isotopes record temperatures of recrystallization in deeply-buried dolomitic reservoirs, through comparison with fluid-inclusion data. The studied reservoir is the Cretaceous Pinda Formation, offshore Angola, a deeply-buried dolomitized sedimentary carbonate hydrocarbon reservoir. It provides an ideal test case as samples from industry wells are available over a relatively wide burial depth range of c. 2000–4000 m below seafloor (mbsf) and the constituent dolomites are relatively homogeneous. Across this depth range, fluid-inclusion homogenization temperatures for the Pinda Formation record a range of temperatures from c. 110 to 170°C, increasing with depth. These closely match present-day ambient well temperatures, indicating recent resetting of the fluid inclusions. Clumped isotopes, however, record temperatures significantly ( c. 20–60°C) below fluid-inclusion and well temperatures for the seven samples analysed. The deepest five samples ( c. 2800–3700 mbsf) record clumped isotope temperatures of around 100–120°C, interpreted to represent a deep burial recrystallization event responsible for a massive (re)dolomitization of the reservoir. The lower clumped isotope temperatures (65 and 82°C) of the shallower (2055 and 2740 mbsf) samples are interpreted to represent physical mixing of two dolomite generations due to incomplete burial recrystallization of an early shallow dolomite. Determination of temperature through clumped isotopes allows calculation of the parent fluid δ 18 O values. In the five deepest samples, the fluid δ 18 O values of 3.7–6.5‰ cluster around the modern-day porewater composition (5‰), suggesting that burial dolomitization occurred in the presence of evolved brine. Mineral δ 18 O values of c . −7 to −4.5‰ are lower than pristine Cretaceous marine dolomite and are in accordance with burial recrystallization. Clumped isotopes are therefore interpreted to record temperatures corresponding to open-system burial recrystallization events. This study shows that clumped isotopes are a valuable tool in characterizing the thermal history of deeply-buried (>2000 m) carbonate hydrocarbon reservoirs. Supplementary material: All standard and sample data are available at https://doi.org/10.6084/m9.figshare.c.3945184

Abstract During the Early Cretaceous, the Lucula Sandstone was deposited along the platform margin of the Malongo Subbasin in offshore Cabinda, Angola. This subbasin is one of a series of lake-filled syn-rift basins that formed along the West African continental margin at an early stage of the opening of the Atlantic Ocean. The Lucula Sandstone is subdivided into a shallow-water facies association, deep-water facies association, and dolostone facies. The shallow-water facies association has beach and nearshore facies and fluvio-deltaic facies. Beach and nearshore facies are characterized by fossiliferous, planar- to cross-laminated sandstones arranged in cyclic, upward-fining packages. Fluvio-deltaic facies are characterized by large-scale cross-bedding and deformed, well-sorted sandstones. The deep-water facies association has turbidite- and slump-dominated facies. Turbidite-dominated facies consist of shales and fine-grained, planar-laminated turbidite sandstones. Slump-dominated facies consist of highly bioturbated, very fine-grained sandstones and siltstone with slump structures. Dolostone facies contain sandy, highly recrystallized dolomite. Two orders of cycle frequency occur in the Lucula Sandstone. Low-frequency cycles are represented by the interstratification of shallow- and deep-water cycles. These cycles are generated by longer-term, climate-forced lake-level changes. High-frequency cycles are represented by the cyclic pattern of beach and nearshore facies. Cycles are produced by low-amplitude lake-level changes caused by short-term climatic changes. Sandstones of the beach-nearshore and fluvio-deltaic facies have the best reservoir quality in the Lucula Sandstone. The distribution of potential shoreline and deltaic reservoirs like the Lucula Sandstone is generally predictable with use of depositional models of rift lake systems. But because of the complex nature of rift systems, stratigraphic prediction will always be somewhat problematic.