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Unveiling the Fluid History of Unconventional Reservoirs Via Diagenetic Fracture-Fill Studies: An Example From the Avalon and Wolfcamp Formations, Delaware Basin


In the absence of coarse diagenetic cements, understanding the fluid history of mudrocks is challenging. Fracture-fills provide an opportunity not only to evaluate the diagenetic history of a given reservoir, but also, through the study of fluid inclusions, to evaluate the timing and type of hydrocarbons present through the history of the reservoir. Mineralized fractures with fluid inclusions were studied to establish the fluid history and diagenesis; and to provide temperature and fluid quality data for given geological periods. The earliest veins, the bedding-parallel (‘beef’) veins filled with antitaxial fibrous calcite, occur primarily in the organic-rich (TOC>3%) mudstone facies of Wolfcamp Fm and are absent in the Avalon Fm. ‘Beef’ veins contain one-phase, non-fluorescent primary inclusions that might point to low temperature (<50°C) precipitation of calcite with no liquid HCs present. Present-day pressure data indicate the overpressured interval coincides with the highest ‘beef’ vein density. This may point to long-lasting overpressure in this zone. Thus the ‘beef’ veins in the Wolfcamp Fm are good indicators of the overpressured, organic-rich mudstone facies. Prior paleomagnetic study suggests the most common vertical/sub-vertical fractures are of WNW-ESE strike and Late Permian in age. These thin veins are sharp-walled with no evidence for lateral migration of the fluids towards the host rock. The veins are predominantly filled with deformed Ba-Sr sulfates and Ca-Mg-Fe carbonates containing primary aqueous inclusions with a Th range of 50-70°C and with a salinity of 18-20 NaCl equ. wt%. Primary petroleum inclusions entrapped in the same minerals, suggest presence of light HC as early as in the Late Permian. The last vein-filling mineral is the non-deformed calcite, precipitated from a higher temperature (Th=90–120°C) and lower salinity (11 NaCl equ. wt%) fluid. This fluid migration phase is either related to the Laramide Orogeny, representing the maximum burial stage of the studied formations, or it is related to the Basin and Range uplift period. Mineralized fractures with similar fluid inclusion and stable isotope data were documented (Barker & Halley 1986*) from an outcrop located miles distant from the well location. Similar fluid history over large distances might indicate regional fluid flow systems that resulted in cementation along fractures. *Barker & Halley 1986, Soc. of Econ. Paleontologists and Mineralogists Spec. Publ.38, 189-203.