uStratigraphic framework

uFigure captions

uFacies

  uChert-free

  uChert-bearing

uGas reservoirs

uPorosity development

uStratigraphic framework

uFigure captions

uFacies

  uChert-free

  uChert-bearing

uGas reservoirs

uPorosity development

uStratigraphic framework

uFigure captions

uFacies

  uChert-free

  uChert-bearing

uGas reservoirs

uPorosity development

uStratigraphic framework

uFigure captions

uFacies

  uChert-free

  uChert-bearing

uGas reservoirs

uPorosity development

uStratigraphic framework

uFigure captions

uFacies

  uChert-free

  uChert-bearing

uGas reservoirs

uPorosity development

Figure Captions

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Facies 

Chert-Free Facies 

The chert-free facies includes three main lithologies: 1) sandstone and sandy skeletal packstone, 2) fine-to very coarse-grained skeletal packstone, and 3) fine- to very coarse-grained skeletal grainstone. The sandstone and sandy packstone of the Cable Canyon Formation, ranging from 2-20 feet thick, is a nearshore deposit characterized by well rounded, poorly sorted, and fine to very coarse quartz sands and skeletal fragments. The skeletal packstone of the Lower Upham is a dark-colored, poorly sorted, massively bedded shallow-marine deposit that accumulated rapidly; it consists of poorly sorted coarse-grained, diverse open-marine-fauna. The sandstone, sandy packstone, and skeletal packstone form the base of Sequence I’s LST. The lighter colored chert-free packstones of the Upper Upham and Lower Aleman, characterized by a coarser-grained texture with diverse skeletons, are offshore bank deposits occurring as the lower section of the HSTs. The chert-free skeletal grainstone facies, especially the cross-bedded, coarse- to very coarse-grained skeletal grainstones (Figure 5 A) represent progressive shoal deposits that cap the HSTs. (Figures 2, 3, 4, and 6).

Chert-Bearing Facies 

The chert-bearing facies consist of host limestone and chert nodules, bands, and layers. The three primary lithologies are: 1) fine-grained skeletal grainstone (Figure 5 B), 2) fine-grained echinoderm packstone (Figure 5 C) and 3) lime wacke/mudstone; all are significant TST components. The wacke/mudstone found in the TSTs of the Upham, Lower Aleman, and Lower Cutter are dark in color and contain very fine spicules and spiculitic chert, representing deeper water, lower energy (deep-ramp) deposition with maximum water depth during the 3rd-order sequence sea-level rise (Figures 2 and 3). 

The fine-grained skeletal grainstones and packstones deposited in the upper Aleman are characterized by an even lighter color and well-sorted skeletons (dominantly echinoderms and ostracods). This is interpreted to be a medium-energy bank deposit capping a TST high-frequency sequence (Figure 6). The remnant structures in the chert coincide with that of the surrounding host rock. The Upper Aleman’s lithological vertical assemblage and lateral distribution is interpreted as a shallow-marine skeletal bank deposited on a gentle ramp developed toward the south. The HST coarser grained skeletal grainstone capping each cycle could have been partially exposed to the northeast (Figure 4).

Gas Reservoirs 

Three gas-reservoir intervals, Cutter, Aleman and Upham, have been drilled and reportedly developed. The reservoir porosity in the Cutter Formation in the northern portion of the study area occurs primarily in the dolomite that developed within the HST skeletal grainstone with some minor contribution from porous and fractured chert. The Upham reservoir tested in the southern part of the study area included porosity at the top of the HST grainstone and fractures in the transgressive chert. The upper Aleman is the primary Montoya pay and contains a number of high-frequency sequences and high-frequency sequence sets of TST chert-bearing and HST chert-free limestones (Figures 2 and 6). The reservoir porosity is predominantly from the chert. Reduced interparticle (Figure 5 D), moldic (Figure 5 E), small pore and micro (Figure 5 F) porosities in the chert with some minor porosity developed in the dolostone and limestone provide the primary gas reservoir.

Sedimentary History and Porosity Development 

During deposition of the Montoya, a vast deep siliceous ocean covered the area to the south. The movement of the silica-bearing upwelling water from south to north along the sea floor resulted in early silicification that was influenced by the environments of deposition, relative sea level, and sedimentary facies. The development of the chert and chert porosity was a direct result of the reaction rates and products of interstitial water and sediments. The LST sandstone and packstone of the Cable Canyon and Lower Upham were deposited in a relatively restrictive shallow-marine environment during a period of rapid sediment accumulation, which highly limited the upwelling activity, resulting in a chert-free facies. During the progressive HST grainstone deposition, high-energy waves, as well as storm and tidal currents forced the nearshore, higher-temperature, higher-salinity/lower-CO2-content water deeper. The interstitial water of the shoal deposits was saturated with CaCO3 but unsaturated with SiO2, resulting in other chert-free facies of the Aleman and Upham. The transgressive packages of Upham, Aleman and Cutter were deposited on the ramp and skeletal bank during sea-level rise. The relatively lower energy and deeper water resulted in organic- and carbonate-mud-rich deposits. The relatively higher CO2 partial pressure of the deeper water and organic acid from decomposition can decrease the pH value of the interstitial water in the sediments, enhancing silicification. The porosity development was strongly related to the texture of the precursor sediment along with the intensity and rate of silica diagenesis. The porous chert of the Montoya pay occurs only in the skeletal-bank facies. The reduced interparticle porosity, representing the dominant pore system has been created from silicification of original interparticle spaces in the grainstone and dissolution of interparticle matrix in the packstone. This initial silicification occurred during TST deposition and shallow burial (before compaction). 

Three stages of silica diagenesis controlled the porosity evolution. The first stage was dissolution of metastable matrix and skeletal fragments as the siliceous, low-pH, upwelling water began to replace the primary interstitial water. The dissolution of this stage enlarged interparticle pore spaces and created moldic porosity. The second stage is silica precipitation on the pore walls to form a silica rim that partially replaced the metastable grains, such as echinoderms, ostracods, brachiopods, and bryozoans resulting in the main porous reservoir chert in the Aleman. The third stage was continuous silica precipitation and replacement that completely filled the porosity and replaced the calcite, forming a non-porous chert. The subsequent HST limestone deposition restricted further silicification and protected the porosity from further chert precipitation. In some areas the silicification was physically restricted in the second stage, resulting in reduced interparticle porosity. Elsewhere the chertification was almost completely prevented between stage 2 and stage 3, forming a small pore system.

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