--> Abstract: Study of Calcite Cement in Submarine Fan Complex, South Wells Member of the Cherry Canyon Formation, Delaware Mountain Group, Western Delaware Basin, by Saswati Chakraborty and Robert Trentham; #90089 (2009)

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Study of Calcite Cement in Submarine Fan Complex, South Wells Member of the Cherry Canyon Formation, Delaware Mountain Group, Western Delaware Basin

Saswati Chakraborty and Robert Trentham
The University of Texas of the Permian Basin

Calcite cement is found to be distributed throughout the reservoir quality sandstones in four cores (~750ft) of the South Wells member of the Cherry Canyon Formation in the Ford Geraldine Field area in the Delaware Basin, along the Reeves-Culberson County line, Texas. Fine grained, well sorted subarkosic Cherry Canyon sandstones to coarse siltstones with silica & calcite cement and authigenic clay matrix are deposited in a sub-marine channel complex. In this high quality reservoir rock with 22 to 28% intergranular porosity, silica cement and authigenic clay are uniformly distributed. However irregularly distributed calcite cement ranges from 0 to 28% of the total volume, with associated reduction in porosity from 28 to 3% in some cases. Therefore the presence of calcite as cement is a critical component in determining fluid flow within the reservoir.

The calcite cement occurs in the form of tightly cemented zones that contrast markedly from the surrounding silica and clay cemented sandstones. The calcite cements are characterized according to shape: blocky, elongated spike-like bodies and elliptical zones. The most abundant cemented intervals are the blocky zones which are 2” to 12” thick, whereas spikes have visible terminations in core and are less than 1” thick and elliptical zones with displacive texture and a maximum diameter of 2”. An interesting corollary is that thicker, partial Bouma Sequences (mostly Ta) have fewer cemented zones than thinner sequences. The distribution of blocky calcite cemented zones is irregular and would appear to be unpredictable. They are however somewhat more abundant near the top of individual partial Bouma Sequences.

Within the South Wells, there are four channel intervals which are separated by thick (~15’ to 18’) condensed sections of mudstone and siltstone. Within each channel interval are 1 to 6 Bouma sequences which are separated by very thin intervals of siltstone or mudstone filled scoured surfaces. The average percentage of sandstone, laminated siltstone and mudstone are 62%, 28% and 10% respectively in the four cores and the tightly calcite-cemented zones form an average of 6-7% of the total sandstone-siltstone reservoir interval of these cores. The calcite cements therefore is probably a dominant control on heterogeneity in porosity and permeability distribution.

This study documented calcite cement distribution and morphology in the sands in an attempt to predict the lateral extent of these cemented reservoir intervals by correlating the widely spaced well data in the submarine fan complexes. Prediction of the distribution of these calcite cemented, non-reservoir intervals is an important step in reservoir characterization because their irregular distribution influences both the path of fluid flow and reservoir compartmentalization. The calcite-cemented zones appear to be capable of acting as either barrier or baffles to flow within the reservoir. The blocky cemented intervals are correlatable with thin low porosity streaks on sonic logs but have no associated character on gamma-ray logs where as the laminated siltstone to mudstone intervals between Bouma sequences have some gamma ray character and less distinct sonic log responce.

Highly localized hydrologic factors appear to be is one of the determining factors of the spatial distribution of pore-filling calcite. It has been proposed for the Ramsey Sands in the Ford Geraldine Fields that there is an external source of calcite from carbonate debris and carbonate rock fragments in associated distal basinal siltstones and limestones units. However, evidence from fossils dissolution and the presence of large carbonate rock fragments in the Cherry Canyon suggests that carbonates deposited within the channel sands could be a potential source of calcite cement. The presence of potential barriers and baffles to fluid flow within most of the incomplete Bouma Sequences suggest highly complex reservoir geometries which will potentially impact primary, waterflood, and CO2-flood recoveries.

AAPG Search and Discovery Article #90089©2009 AAPG Southwest Section Meeting, Midland, Texas, April 26-29, 2009