BEYOND CONVENTIONAL: GLEANING DEEPER INSIGHTS INTO DOLOMITIZATION OF THE BAKKEN PETROLEUM SYSTEM THROUGH IN-SITU C- AND O-ISOTOPE MICROANALYSIS

Recent advances in carbonate δ¹³C and δ¹⁸O micro-analysis by secondary ion mass spectrometry (SIMS) permit detailed study (1-10 µm scale) of the often multi-staged dolomitization histories of sedimentary rocks. Avoiding the signal-averaging effects of conventional isotope measurements allows for clearer insights into the environments and post-depositional processes responsible for different episodes of dolomite formation.

The middle member of the Late Devonian / Early Mississippian Bakken Formation (Williston Basin) is a tight-oil reservoir that consists largely of dolomitic siltstone and fine-grained sandstone stratigraphically bounded by two organic-rich black-shale beds. This sequence was gradually buried ~3.5 km and heated to 165°C in the deep part of the basin. Bulk isotope analysis of calcite that occurs as an early cement (pre-compaction) in un-dolomitized sediment lenses (cm-m scale) yields values consistent with precipitation from contemporaneous seawater at 25-30°C (δ¹³C: 1.5 to 4.5‰, δ¹⁸O: -3.5 to -5.5‰ VPDB). Isotope micro-analysis of chemically zoned dolomite crystals (variable Fe/(Mg+Fe) ratio of 0.0-0.4) reveals consistent core-to-rim trends. To a first order, average δ¹³C and δ¹⁸O values respectively decrease from 0 to -9‰ and -5 to -8‰ VPDB. Values of dolomite cores and their initial ferroan rims are spread across a range of 4‰, a pattern consistent with growth in a dynamic, near-surface environment. The remaining six concentric growth zones record δ¹³C and δ¹⁸O values that: 1) are strongly correlated and less variable (<0.5‰) within any one zone and 2) tend to decrease rimward, in turn consistent with growth under steadily changing temperature and pore-water conditions expected of deeper burial. Post-dolomitization extension fractures are filled in part by calcite (δ¹³C: -2.5 to -4.5‰, δ¹⁸O: -4.5 to -6.5‰) with fluid inclusions indicating growth entrapment at 110-115°C (USGS), thus placing a tentative upper limit on the temperature and timing of dolomitization (early Cretaceous based on burial history). The range of isotope values measured in both carbonate phases also suggests that all stages of dolomitization and subsequent fracture-sealing occurred in the absence of significant volumes of dissolved carbon derived from organic maturation in adjacent shale beds.