Abstract

The Grayburg Formation is a prolific hydrocarbon-producing interval in the Eunice Monument South Unit (EMSU) along the northwest portion of the Central Basin Platform (Figures 1–3). The Grayburg outcrops in several canyons in the Guadalupe Mountains, 100 miles west of EMSU, and is analogous to the productive Grayburg reservoirs along the northwest margin of the Central Basin Platform.

The Grayburg Formation was deposited upon a carbonate ramp as a series of shallowing-upward fifth-order parasequences of interbedded carbonates and siliciclastics (Figure 4). These parasequences consist of a dominant carbonate component and a subordinate siliciclastic component. Sandstones were deposited in some but not all parasequences. Carbonate rocks are composed of ooid dolograinstones to mud-poor dolopackstones and were deposited in a high-energy shoal complex. Landward, the carbonates are composed of mud-rich dolopackstones to particle-poor dolowackestones and were deposited in a low-energy back-shoal setting.

Dolomitic sandstones (subarkoses to calclithites) were deposited at the base of individual parasequences in the offshore shallow marine, and occasionally in lower shoreface environments, as slightly bioturbated, calcareous sandstones. They are overlain by the grainstone to mud-poor packstone shoal facies or mud-rich packstone to wackestone back-shoal facies, which form the upper part of individual shallowing-upward parasequences. Where sandstones were not deposited, low-energy mudstones, particle-poor to particle-rich wackestones and, to a lesser extent, mud-rich packstones form the base of parasequences.

Reservoir-quality porosity in the dolostones is preserved or enhanced by a combination of parameters: 1) three primary depositional characteristics-bed thickness, grain richness, and grain size; 2) dissolution during subaerial exposure; 3) dolomitization (early diagenesis); and 4) dissolution during deep burial (late diagenesis). The best reservoir facies are the dolograinstones and mud-poor dolopackstones where they are thick to massively bedded and well-sorted. Thinner bedded, mud-rich dolopackstones, particle-rich dolowackestones and particle-poor dolowackestones contain progressively lower reservoir porosity in an updip direction, eventually forming a lateral seal (stratigraphic trap).

Porosity within oil-saturated dolomitic sandstones was created by dissolution of feldspars (secondary porosity). The permeability of these sandstones is low due to the high percentage of dolomite matrix and authigenic clay in the secondary pores. These sandstones form baffles to the vertical and lateral migration of fluids. Updip, these sandstones contain less porosity and permeability, are not oil saturated, and form barriers to vertical and lateral migration fluids.

Shallowing-upward parasequences are thicker downdip, where they were deposited in a shoal environment. Updip, these parasequences are thinner and more numerous where they were deposited in a back-shoal environment. Bifurcation of parasequences was recognized in cored wells spaced one mile apart, and in outcrop when measuring stratigraphic sections, on a scale simulating one, five, ten twenty and forty acre spacing units. Outcrop studies help to better understand subsurface interwell heterogeneities on a bed-by-bed and parasequence-by-parasequence basis, that could not be recognized from subsurface data alone.

Dolomitic sandstones can be correlated through all of the canyons in the study area in the Guadalupe Mountains and are excellent marker beds. They can also be utilized in the subsurface at Eunice Monument South Unit as marker beds in well log correlation.

By utilizing reservoir characteristics, as they were understood in EMSU, outcrop studies of equivalent facies in the Guadalupe Mountains and applying sequence stratigraphic interpretation to the data has greatly enhanced reservoir management during initial waterflood fluid injection and reservoir fillup. For example, early water break through was experienced in EMSU well number 119, in the northern end of the unit (Figure 5). In this area a high porosity, high permeability bed of mud-poor dolopackstone to dolograinstone within one parasequence was connected with EMSU injection well number 108, located directly to the north. This particular injection well was taking water on a vacuum. Once reservoir connectivity was understood the highly porous permeable bed of mud-poor dolopackstone to dolograinstone in the production well was successfully squeezed off. The injection well immediately built-up pressure, indicating that the waterflood had been redirected elsewhere through this porous interval. Later, the oil bank that was formed, due to the successful workover, arrived at EMSU well number 109.