Diagenesis of the palaeo-oil-water transition zone in a Lower Pennsylvanian carbonate reservoir: Constraints from cathodoluminescence microscopy, microthermometry, and isotope geochemistry

• Published in: Marine and petroleum geology Vol. 72; pp. 45 - 61
• Main Authors: Kolchugin, A.N., Immenhauser, A., Walter, B.F., Morozov, V.P.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.04.2016
• Subjects: Carbonate reservoirs; Carbonates; Cementation; Computational fluid dynamics; Diagenesis; Fluid flow; Fluids; Geochemistry; Hydrocarbons; Marine; Migration; Reservoirs; Carbonate reservoirs; Geochemistry; Hydrocarbons; Diagenesis
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2016.01.014

Oil-water transition zones in carbonate reservoirs represent important but rarely studied diagenetic environments that are now increasingly re-evaluated because of their potentially large effects on reservoir economics. Here, data from cathodoluminescence and fluorescence microscopy, isotope geochemistry, microthermometry, and X-ray tomography are combined to decipher the diagenetic history of a 5-m-long core interval comprising the oil-water transition zone in a Lower Pennsylvanian carbonate reservoir. The aim is to document the cementation dynamics prior, during, and after oil emplacement in its context of changing fluid parameters. Intergrain porosity mean values of 7% are present in the upper two sub-zones of the oil-water transitions zone but values sharply increase to a mean of 14% in the lower sub-zone grading into the water-saturated portions of the reservoir and a very similar pattern is observed for permeability values. In the top of the water-filled zone, cavernous porosity with mean values of about 24% is found. Carbonate cements formed from the earliest marine to the late burial stage. Five calcite (Ca-1 through 5) and one dolomite (Dol) phase are recognized with phase Ca-4b recording the onset of hydrocarbon migration. Carbon and oxygen cross-plots clearly delineate different paragenetic phases with Ca-4 representing the most depleted δ13C ratios with mean values of about −21‰. During the main phase of oil emplacement, arguably triggered by far-field Alpine tectonics, carbonate cementation was slowed down and eventually ceased in the presence of hydrocarbons and corrosive fluids with temperatures of 110–140 °C and a micro-hiatal surface formed in the paragenetic sequence. These observations support the “oil-inhibits-diagenesis” model. The presence an earlier corrosion surface between phase Ca-3 and 4 is best assigned to initial pulses of ascending corrosive fluids in advance of hydrocarbons. The short-lived nature of the oil migration event found here is rather uncommon when compared to other carbonate reservoirs. The study is relevant as it clearly documents the strengths of a combined petrographic and geochemical study in order to document the timing of oil migration in carbonate reservoirs and its related cementation dynamics. •Very detailed petrographic data set providing insight in oil migration patterns and fluid properties.•Oil migration is limited to one paragenetic phase, a rather unusual feature.•The data set supports the “oil-inhibits-diagenesis-model” but also specifies boundary conditions.•Data on the subsurface Carboniferous from Russia are extremely scarce in the open literature.