Type log though the Yates, Tansill, Salado, and Alibates Formations. Terry 16, Mobil Oil Corporation No. 1 Texas Tech University.
The Salado Formation is the dominant halite-bearing unit of the Midland Basin and was mapped in detail for this study. Based on a model of salinity-controlled anhydrite-halite-mudstone depositional cycles (Hovorka, 1994), I used anhydrite beds as the major stratigraphic markers. Anhydrite represents the most-flooded, least-restricted conditions over the evaporite shelf where wind, storm, and seasonal circulation was adequate to maintain gypsum deposition.
Anhydrite beds are recognized by low response on gamma-ray logs, normal bore-hole diameter on caliper logs (in contrast, halite is commonly strongly embayed because it is dissolved in contact with undersaturated drilling mud), high count on neutron logs, high velocity on sonic logs, and high density log response. Anhydrite is typically fairly pure, although bed thickness limits log response from attaining the theoretical values for the thinner beds. Each anhydrite bed was flagged, correlated, and numbered. Regionally traceable beds were numbered 20, 30, 40, 50, and 60, and beds of more local extent were assigned intervening numbers (number 10 and 15 were used to subdivide Tansill stratigraphy, and 80 and 90 were used for anhydrite beds in the overlying Alibates Formation). Anhydrite bed 20 was identified across the entire study area and is distinctive because, in most areas, a thin insoluble residue of mudstone occurs at the base. Overlying anhydrite beds pinch out toward the basin margins or are included in insoluble residue where intervening halite has been dissolved.
Anhydrite in the Salado Formation is commonly partly replaced in some intervals by polyhalite (Na2MgK2(SO4) 4 ´ H2O). In core in the Palo Duro Basin and the Delaware Basin, polyhalite is observed to occur as needles and fine-grained masses that are typically red or pink because of thin iron-oxide coatings on polyhalite crystals. It is an early diagenetic replacement of gypsum as a result of interaction with pore water in the subaerial or shallow burial environment. The distribution of polyhalite is irregular on a fine scale, where it forms fabric-specific replacement textures and nodules, and on an intermediate scale, where it may replace only the floors of large polygons (Robert Holt, IT Corporation, 1990, personal communication), as well as on a regional scale. Although polyhalite is mined commercially as a potassium source in the Delaware Basin east of Carlsbad, New Mexico, no commercial uses are noted in the Midland Basin.
Polyhalite produces a strong gamma-ray-log response. Polyhalite has relatively low solubility in brine, so polyhalite beds are intervals of normal hole size on caliper logs, although thin beds within salt are commonly mechanically broken. Neutron-log response is variable because common admixture with anhydrite offsets the log response to the hydrous mineral. Polyhalite is admixed with mudstone in some settings, and these are also difficult to accurately separate.
Bedded halite is the most common lithology in the Salado Formation. In cores from adjacent basins (Lowenstein, 1988; Hovorka, 1990; 1994), bedded halite contains 5 to 15 percent anhydrite and mudstone as disseminated impurities and as millimeter- to centimeter-thick laminae. Log response and cycle structure suggests that halite in the Midland Basin probably has similar composition and fabric. Halite is identified in logs by a low gamma-ray response similar to anhydrite, oversized hole on caliper log, variable moderate-low neutron response, moderate and variable density and sonic log response, and high resistivity. In boreholes drilled with halite-saturated brine, halite beds produce little or no caliper log deviation.
Bedded halite is transitional into mudstone-halite mixtures and into mudstone. Mudstone in cores from the Palo Duro Basin (Hovorka, 1990; 1994) is composed of subequal mixtures of arkosic silt and illite-montmorillonite-dominated clays. Mudstone-halite mixtures or ýchaotic mud-saltţ (Handford, 1982) are beds composed of poorly or nonbedded mixtures of euhedral or corroded halite crystals and mudstone matrix. Mudstone-halite mixtures are transitional into mudstone beds with minor inclusions of halite as euhedral or corroded halite crystals. Mudstone beds in turn are transitional by inclusion of less clay into siltstone and very fine sandstone. All these fine-grained clastics are collectively known as siliciclastic red beds.
Mudstone and mudstone-halite beds form during periods of prolonged exposure of the halite flat (Fracasso and Hovorka, 1986; Hovorka, 1994). Siliciclastics are transported onto the flat by sequential dust storm transport of fine materials, reworking by rainfall, and reworking by marine-derived saline-storm floodwater. Exposure and water-table drop cause formation of karst pits in halite, and these pits are filled with mudstone and mudstone and halite mixtures. The resulting distribution of mud is heterogeneous on a fine scale because pit fillings may be several feet thick adjacent to areas between pits where mudstone is thin or missing.
Log response to siliciclastic intervals is characterized by higher gamma-ray-log response than anhydrite and halite, and distinctly low neutron-log response because of high clay lattice and capillary water content. Sonic-log response is also generally low. Permeability of mudstones is generally considered to be very low because of high clay content; siltstone and sandstone porosity is typically occluded by halite cement, although investigation of the extent to which these generalities are true at a site scale may be needed. Borehole size as shown by caliper-log response in siliciclastic red-bed intervals is variable depending on drilling conditions and mud composition; in some boreholes, mudstones, and even siltstones and sandstones, are as strongly washed out as halite; in other boreholes, many siliciclastic beds form smaller borehole diameters than adjacent halite. Log suites were not adequate to consistently separate mudstone-halite mixtures from mudstone beds or mudstone beds from silty or sandy siliciclastic red beds.
The Tansill Formation is a highly cyclic and laterally heterogeneous unit about 100 ft thick. Toward the Delaware Basin, the Tansill Formation is dominated by anhydrite with or without dolomite and siliciclastic interbeds. In depositional updip environments toward the east and north margins of the Midland Basin, the Tansill Formation is composed of halite with abundant siliciclastic interbeds. In the middle of the Midland Basin, the basal part of the Tansill Formation is dominantly anhydrite or dolomite, siliciclastics with halite interbeds becoming more dominant upward. The log character of the Tansill is distinguished from the overlying Salado Formation because it contains more thin cycles and more abundant thin siliciclastic beds. Because of the cyclic nature of the sediments, however, no adequate stratigraphic marker was identified to regionally map the Tansill separately from the Salado Formation.
Units and Type Logs
Salado Insoluble Residue, Alibates, and Dewey Lake Formations