Skip to Main Content

abstract

The Permian (Leonardian) Bone Spring Limestone is a significant oil reservoir rock in the Delaware Basin of west Texas and southern New Mexico. Analysis of fluid inclusions trapped in fracture-filling cements, oxygen isotope composition of the cements, and thermal maturity measurements of host rock provide a detailed record of the thermal and fluid history for this area of the western Delaware Basin. Although these cements have petrographically distinct multiple zones, homogenization temperatures (Th) of oil fluid inclusions and δ18O data divide these cements into two (early and late) groups. Early cements have δ18O near O%c and contain dull- blue fluorescent oil inclusions whose Th is mainly between 55°C and 90°C. The late cements occur in fractures that crosscut the early cements and are strongly oriented subparallel to the local structures formed during the Laramide orogeny and Basin and Range uplift. The late cements have δ8O between −8%c and −9%c and contain sparse oil inclusions similar to those in the early cement. Primary fluid inclusions in the late cements have a Th mainly between 90°C and 135°C. Petrographie observations suggest that overlap in Th between early and late cements is due to: (1) incorporation of fragments of early cements into crosscutting late cements; (2) local neomorphism of early calcite along margins of late calcite; and (3) formation of secondary inclusions (some distinguished by yellow fluorescence) during uplift and exposure.

When interpreted through burial history reconstruction, Th and δ18O data suggest that oil migrated relatively early while this rock was about 55°C to 75°C. These temperatures were attained at near maximum burial depth of 1-2 km (estimated 30°C/km paleogradient) at the end of the Permian. The average Th values of primary inclusions of about 110°C are consistent with maximum temperatures of ca. 120°C interpreted from a mean vitrinite reflectance (Rm) of 0.7%, suggesting that the late cements record the maximum temperature attained in the rock. The change in δ18O of about 8%c from the early to late cement is nearly that expected from the difference in Th between these cements. Although maximum burial depth was reached near the end of the Permian, maximum temperature was not recorded until geothermal gradients apparently increased in the Tertiary. The evidence for this higher geothermal gradient is recorded in cements from fractures formed during early Tertiary deformation.

Calculations from calcite/water fractionation relations and Th indicate that early calcite cement precipitated from water that had δ18O of about +9%c and later cements precipitated from water of about +7.5%c. These values are typical for waters remaining after intense evaporation or for deep basin brines. Temperature determinations are associated with sufficient error that both cement generations may have been precipitated from the same water, and cement isotopic differences may have resulted solely from temperature changes.

You do not currently have access to this chapter.

Figures & Tables

Contents

GeoRef

References

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal