Porosity gain and loss in unconventional reservoirs: Example of rock typing in Lower Cretaceous hemipelagic limestones, SE France (Provence)

Abstract

We characterize porosity evolution in hemipelagic limestones of the Lower Cretaceous Vocontian Basin (Northern Provence, SE France), which are surface analogs for deep unconventional reservoirs. Firstly, a structural and sedimentologic analysis was conducted on outcrops and on borehole cores of Berriasian to Lower Valanginian rocks. Secondly, 330 plugs were analyzed for facies, pore typing, porosity and acoustic Vp velocity measurements. This petrophysical dataset was compared to the burial-uplift history of the host rock.Hemipelagic carbonates include four sedimentary formations comprising nine facies splitted into mud- and grain-dominated formations which display 0–8% porosity range depending on the initial grain to mud ratio of the sediments and on basin stress regime evolution during burial-uplift history. Initial porosity was better preserved in grain dominated formations where early micritic cements built bridges between grains, generating a solid skeleton that prevented/inhibited the burial compaction effects. In mud dominated facies, micrite was initially diffuse within the rock matrix but was progressively rearranged under burial stress, while the generation of an extensive amount of stylolites occurred. The differential response of compacted grain and mud dominated facies to a late regional 1 km uplift resulted in large porosity generation (should range from 15 to 35%) localized in the mud dominated facies through the dilatancy of stylolite and pressure-solution seams.This study highlights how important is the combination of classical approaches including facies, pore type and diagenesis characterization with the structural analyses. One key result applicable to several types of unconventional reservoirs is the effective porosity generated by reactivation of compaction structures.