The Bone Spring Formation (Early Permian, Leonardian) of Lea County, New Mexico, was studied in the Gulf Lea “YH” State No. 4 well, in cores from 9177 to 9334 ft depth. The cores are composed of laminated dolomitic mudstones that were deposited on the slope of the Delaware Basin margin. Massive debris flow deposits comprise part of the cored interval. They consist of shelf-derived and slope-derived dolomite clasts in a matrix of dolomudstone. Burial diagenesis has altered the dolomite and modified the porosity within the debris flow deposits to a greater extent than within the slope deposits.
Several stages of diagenesis can be inferred for the debris flows. The “early” stage consisted of matrix dolomitization. The “intermediate” stage started with leaching and fracturing during times of high rates of sedimentation. Intermediate-stage dolomite cemented the secondary pores and replaced matrix carbonate. This dolomite was partly silicified while opal sponge spicules were dissolving in the enclosing slope sediment. The early and intermediate stages of diagenesis occurred during the Late Permian. At that time, the Bone Spring was descending beneath an accumulating 2400 m thick section of siliciclastics, carbonates and evaporites.
The “late” diagenetic stage occurred since the Late Permian while the Bone Spring was at maximum burial depths (approximately 3050 m). This stage includes minor amounts of anhydrite cement and subsequent, very coarse dolomite. Poikilotopic calcite is in some late-stage fractures where it is pseudomorphic after anhydrite. This phase was the last event to affect the Bone Spring. Oil generation and migration have been ongoing processes since the Late Permian; oil entered the section after the anhydrite precipitated.
The three dolomite stages can be differentiated readily on the basis of isotopes and petrography. The intermediate stage exhibits fluorescent zones and aqueous inclusions; the late stage dolomite has prominant cleavage, zoned luminescence, and fluorescent hydrocarbon inclusions. The major porosity-filling phase is the intermediate dolomite. Thus, the debris flows were mostly non-porous and impermeable by the Late Permian, just prior to oil generation.
