U-Pb dating of calcite cement and diagenetic history in microporous carbonate reservoirs: Case of the Urgonian Limestone, France

Abstract

Microporous carbonates can constitute excellent hydrocarbon reservoirs if their micropore and/or nanopore structure is sufficiently developed and continuous. In such deposits, assessing the exact timing of reservoir property stabilization is critical to better understand the postdepositional processes favorable to the creation or preservation of porosity. However, placing reliable and accurate chronological constraints on the formation of microporosity in these reservoirs is a major challenge. In this study we performed absolute U-Pb dating of calcite cements occurring in the Urgonian microporous limestone (northern Tethys margin) of southeastern France. U-Pb ages ranging between 96.7 ± 4.9 Ma and 90.5 ± 1.6 Ma were obtained on the major calcitic phase responsible for the cementation, and therefore the stabilization of microporosity, indicating that this diagenetic process occurred synchronously at the regional scale following an extended subaerial exposure. Our results show that (1) the mineralogical stabilization process responsible for the formation of an excellent pervasive microporous network took place relatively early, and (2) the so-acquired reservoir quality was preserved for more than 90 m.y. These observations emphasize the importance of long exposure periods and associated meteoric influx for the formation and preservation of good microporous reservoirs. DIAGENETIC HISTORY OF THE MICROPOROUS UL, SOUTHEAST FRANCEDiagenetic patterns observed in the UL were described in detail in Léonide et al. (2014), establishing a diagenetic sequence showing a number of well-characterized calcite dissolution and cementation phases. Two main generations of calcite cement, S1 and S2, have been documented in the formation, based on petrographical and textural evidence showing a continuum from the S1 microsparite to the S2 blocky calcite. Note that the S1 phase is too small and inclusion rich to be subsampled for U-Pb dating purposes.The UL is marked by several short-term syn-Urgonian exposures as well as a major regional post-Urgonian subaerial exposure (ca. 3 Ma) related to the Durancian phase (upper Albianlower Cenomanian) (Masse and Philip, 1976). The geographical extension of the Durancian phase, evidenced by a characteristic erosion surface, is closely correlated to the spatial distribution of microporous UL and has been proposed

Direct U-Pb dating of carbonates from micron scale fsLA-ICPMS images using robust regression

“…cements filling the porosity of reservoirs (e.g., Li et al, 2014;Godeau et al, 2018). This is due, among other benefits, to the very good spatial resolution of the method, which allows dating minerals with a surface area of less than one millimeter, to its quickness, allowing to date several samples per day, as well as to the recent availability of carbonate standards to correct matrix-related offsets of U/Pb ratios (Roberts et al, 2017).…”

An Image Mapping Approach to U‐Pb LA‐ICP‐MS Carbonate Dating and Applications to Direct Dating of Carbonate Sedimentation

“…Direct dating of calcite and other carbonate minerals using the U-Pb system is an important tool to constrain the timing of processes such as carbonate deposition during key intervals of Earth history, carbonate diagenesis, metamorphism, fault activity, hydrothermal mineralization, and ancient weathering episodes (e.g., Babinski et al, 1995;Denniston et al, 2008;Godeau et al, 2018;Hill et al, 2016;Hoff et al, 1995;Jahn, 1988;Moorbath et al, 1987;Rasbury et al, 1998;Rasbury & Cole, 2009;Ring & Gerdes, 2016;Smith et al, 1991;Wang et al, 1998;Woodhead et al, 2010). Important prerequisites to the successful U-Pb dating of carbonates are a closed isotopic system, a homogeneous initial ("common") Pb composition, and a sufficient spread in the parent/daughter ratio.…”

U–Pb dating of calcite in ancient carbonates for age estimates of syn- to post-depositional processes: a case study from the upper Ediacaran strata of Finnmark, Arctic Norway

“…The method provides constraints on the ages of sediment deposition and diagenesis (e.g. Israelson et al 1996;Rasbury & Cole, 2009;Hill et al 2016;Godeau et al 2018;Pisapia et al 2018;Drost et al 2019), fossils (e.g. Rasbury & Cole, 2009;Yokoyama et al 2018;Drost et al 2019) and mineralization along fracture and fault planes (e.g.…”