Late‐stage calcites in the Permian Capitan Formation and its equivalents, Delaware Basin margin, west Texas and New Mexico: evidence for replacement of precursor evaporites

Abstract

Comparison of Upper Guadalupian fore‐reef, reef and back‐reef strata from outcrops in the Guadalupe Mountains with equivalent subsurface cores from the northern and eastern margins of the Delaware Basin indicates that extensive evaporite diagenesis has occurred in both areas. In both surface and subsurface sections, the original sediments were extensively dolomitized and most primary and secondary porosity was filled with anhydrite. These evaporites were emplaced by reflux of evaporitic fluids from shelf settings through solution‐enlarged fractures and karstic sink holes into the underlying strata. Outcrop areas today, however, contain no preserved evaporites in reef and fore‐reef sections and only partial remnants of evaporites are retained in back‐reef settings. In their place, these rocks contain minor silica, very large volumes of coarse sparry calcite and some secondary porosity. The replacement minerals locally form pseudomorphs of their evaporite precursors and, less commonly, contain solid anhydrite inclusions. Some silicification, dissolution of anhydrite and conversion of anhydrite to gypsum have occurred in these strata where they are still buried at depths in excess of 1 km; however, no calcite replacements were noted from any subsurface core samples. Subsurface alteration has also led to the widespread, late‐stage development of large‐ and small‐scale dissolution breccias. The restriction of calcite cements to very near‐surface sections, petrographic evidence that the calcites post‐date hydrocarbon emplacement, and the highly variable but generally ‘light’carbon and oxygen isotopic signatures of the spars all indicate that calcite precipitation is a very late diagenetic (telogenetic) phenomenon. Evaporite dissolution and calcitization reactions have only taken place where Permian strata were flushed with meteoric fluids as a consequence of Tertiary uplift, tilting and breaching of regional hydrological seals. A typical sequence of alteration involves initial corrosion of anhydrite, one or more stages of hydration/dehydration during conversion to gypsum, dissolution of gypsum and precipitation of sparry calcite. Such evaporite dissolution and replacement processes are probably continuing today in near‐outcrop as well as deeper settings. This study emphasizes the potential importance of telogenetic processes in evaporite diagenesis and in the precipitation of carbonate cements. The extensive mineralogical and petrophysical transformations which these strata have undergone during their uplift indicates that considerable caution must be exercised in using surface exposures to interpret subsurface reservoir parameters in evaporitic carbonate rocks.

Extensive, uplift-related and non-fault-controlled spar precipitation in the Permian Capitan Formation

“…While the fluids responsible for precipitating these spars may well have been sourced along faults as indicated by the Budd et al (2013) study, they also penetrated unfaulted formation in the fore-reef slope. The clumped isotope data from this study and Budd et al (2013) are evaluated together, along with previously reported outcrop and core relationship data (Scholle et al, 1992). These results indicate that late stage spar precipitation was extensive, occurred long after deposition and included a significant meteoric fluid component, with cementation likely occurring during uplift and not during burial.…”

“…The associated units experienced burial relatively soon after deposition and remained at a more or less constant depth until~175 million years later when Basin and Range tectonic activity fostered abrupt exhumation (Scholle et al, 1992). Dominant rock types of the Capitan Formation and its equivalents include carbonate boundstones, grainstones, wackestones, packstones and mudstones; the distribution of which are strongly associated with the particular depositional environments listed above (Garber et al, 1989).…”

“…The relative relationships among these phases support the following cement chronologic sequence succession: 1) fibrous aragonite 'marine cements', 2) isopachous fringes, 3) evaporite precipitation, and 4) calcite spar replacement of evaporites. The precipitation of calcite spar is thought to replace precursor evaporites based largely on the common poikilotopic habit of the calcite spars, a common sign of evaporite replacement (Murray, 1960;Clark and Shearman, 1980;Scholle et al, 1992), and the presence of sulfate mineral inclusions within some calcite spars (Harwood et al, 1990;Scholle et al, 1992). Aside from this relative paragenetic progression, much is unknown about the precise relationship of cementation to the overall history of Capitan diagenesis including its burial, uplift and exhumation history.…”

“…2). The conditions associated with the formation of these spars has been discussed in several papers which present different models for their formation based on isotope and elemental data (Mruk, 1985;Meyers and Lohmann, 1985;Scholle et al, 1992). Recent clumped isotope analyses by Budd et al (2013) has demonstrated that spars of the outer-shelf and reef facies of the Capitan Formation formed in the presence of relatively high temperature fluids (up to~100°C).…”

“…1) exhibit a variety of diagenetic cements (Meyers and Lohmann, 1985;Mruk, 1985Mruk, , 1989Scholle et al, 1992;Mazzullo, 1999) including late stage, blocky calcite spars (Fig. 2).…”