Nature and distribution of diagenetic phases and petrophysical properties of carbonates: The Mississippian Madison Formation (Bighorn Basin, Wyoming, USA)
Résumé
The purpose of this study is to characterize factors controlling the nature and distribution of the diagenetic phases that gave rise to carbonate reservoirs. To do so, a pluridisciplinary approach was carried out, integrating sedimentologic and diagenetic studies on the Madison Formation (Lower Carboniferous, 357-340 My) which is a carbonate reservoir in subsurface.The Madison Formation, outcropping in the Bighorn Basin (Wyoming, USA), is a 340 m thick carbonate series composed of four to seven 3rd-order depositional sequences (S1 to S7) depending of the palaeogeographic location. The first three sequences (S1 to S3) were deposited under arid and warm conditions during Tournaisian times which favoured high accumulation of carbonates leading to a morphological change from a quite flat ramp (S1 to S2) to a wide platform (S3). It also probably favoured the early calcite cementation (isopachous and syntaxial calcite cements) of the subtidal deposits and the early dolomitization (D1) of the supratidal to intertidal sabkha ones. In addition, the very flat profile occurring during S3 was also responsible for the postponed dolomitization (D2) of S1-S2 due to reflux of brines, at various degrees depending on the palaeogeographic location. The deposition of S4 to S7 under humid conditions during Visean times were associated with 1) a decrease of the carbonate accumulation and of the dolomitization; 2) dissolution processes at micro- (pores network) and macro-scales (karst, collapse breccia...) and calcite cementation (C1). The spatial distribution of all these first diagenetic phases acts as a controlling factor on the distribution of the later burial diagenetic phases. Thus, the mesogenetic calcite cements are mostly observed in the secondary porosity created by dissolution (karsts, collapse breccias) or dolomitization. The burial diagenesis of the Madison Formation were characterized by 1) a dolomitization phase (D3) in proximal parts of the platform leading to an increase of porosity and permeability and 2) calcite cementations (C2-C3) in the distal parts of the platform leading to a decrease of these properties.The Madison Formation provides a good spatial representation of sedimentary and diagenetic heterogeneities that may occur in the carbonate reservoirs due to various palaeogeographic locations, climates, burial history and structures.