Ahr, Wayne Merrill¹
(1) Texas A&M University, College Station, TX

ABSTRACT: Hydrothermal Dolomite and Hydrocarbon Reservoirs

Hydrocarbon reservoirs and hydrothermal seem incongruous in the same sentence because hydrothermal alteration is a type of contact metamorphism. Hydrothermal silicates are known to have plugged pores in Salton Basin geothermal wells for example. Hydrothermal also refers to heated aqueous solutions that have no demonstrable magmatic affiliations (Hurlbut & Klein, 1977) and workers commonly cite basinal brines as the source of hot solutions involved in deep burial diagenesis. Sulfides, including MVT ores, fluorite, and saddle dolomite commonly occur as exotic minerals in carbonate host rocks. Saddle dolomite is especially common. We found it in Mississippian reservoir rocks (Hardeman Basin, Texas), in Lodgepole reservoirs (Mississippian, North Dakota), in Lodgepole outcrops, Big Snowy Mountains, Montana, and in Jurassic (Smackover - Cotton Valley) reservoirs around the ancestral Gulf of Mexico. Saddle dolomite typically occurs as late-stage cement in vugs, caverns, and fractures. Oxygen isotopic signatures on our saddle dolomites consistently indicated temperatures of crystallization around 60 to over 100 degrees C. In all cases, saddle dolomite was intimately associated with residual hydrocarbons suggesting a causal link between saddle dolomite crystallization and hydrocarbon migration. This link also exists in the famous Blue John fluorite deposits in central England and it may exist in chalcedony replacements of reservoir rocks. Geothermometric indicators from research on contact metamorphism and source rock maturation might refine our knowledge of processes and temperatures during late burial diagenesis.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004