--> Abstract: Sequence Stratigraphic Interpretation of the Leonardian (Permian) Bone Spring Formation, Delaware Basin: Insights on Reciprocal Sedimentation from Three-Dimensional Seismic Data, by B. S. Hart; #90937 (1998)

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Abstract: Sequence Stratigraphic Interpretation of the Leonardian (Permian) Bone Spring Formation, Delaware Basin: Insights on Reciprocal Sedimentation from Three-Dimensional Seismic Data

HART, BRUCE S., New Mexico Bureau of Mines and Mineral Resources

Summary

Like most other depositional settings, existing depositional sequence models of carbonate-siliciclastic shelf, slope and basinal areas have been based primarily on 2-D seismic, outcrop and subsurface log and core control. Although these data sets provide insights into depositional processes, 1-D and 2-D facies successions and systems tract development, there is a growing need to examine stratigraphic units in 3 dimensions in order to truly understand the autocyclic and allocyclic controls on stratigraphic architecture. Three-dimensional (3-D) seismic: data, properly integrated with other data sets, represent a hitherto underexploited data set for sequence stratigraphic analyses.

The Bone Spring Fm (Leonardian) of New Mexico provides an exciting test of the applicability of 3-D seismic data sets to study the sequence stratigraphic development of such an unit. This study is based on the examination of three 3-D seismic data sets and wireline logs (covering approximately 90 km{2}) from a slope setting along the northern margin of the Delaware (Permian) Basin. These data, collected for hydrocarbon exploration and development purposes, permit the identification of lithology and stratal architecture (systems tracts and smaller depositional elements) in 3 dimensions.

In order of increasing stratigraphic age, the following members are informally recognized: First Carbonate, First Sandstone, Second Carbonate, Second Sandstone, Third Carbonate, Third Sandstone. Siliciclastic sediments were deposited during lowstands of relative sea level, whereas carbonates were deposited primarily during transgression and highstands. Only limited (areally and stratigraphically) outcrop exposures of the Bone Spring are available, and so a true understanding of the stratigraphic architecture, and controls thereon, needs to be based on the best subsurface information available.

Along the eastern part of the shelf margin (east-central Lea County), the Bone Spring Fm is a progradational to aggradational system that is locally over 1100 m thick. Approximately 5 km of shelf margin progradation can be documented, at least locally, during Bone Spring time. The Bone Spring and underlying Wolfcampian deposits form an essentially continuous package which prograded over a tectonic unconformity that appears to have developed in late Pennsylvanian time. Compaction induced flexure of shelfal equivalents to the Bone Spring is pronounced where the shelf margin prograded over underlying, structurally induced, topography. Elsewhere, by controlling seafloor relief and (possibly) axes of siliciclastic sediment transport to the shelf margin during lowstands pre-Leonardian relative relief associated with Pennsylvanian tectonic movements had a significant impact on depositional processes and patterns.

Carbonate members (Highstand and Transgressive Systems Tracts) of the Bone Spring form the greatest thickness of the slope deposits, and do not generally show distinct clinoform geometries. Instead, reflections tend to be sub-parallel and drape underlying relief. Based on analogies with studies of modem pro grading systems, it appears that highstand slope progradation resulted from the accumulation of carbonate mud that was supplied from the adjacent shelf. Evidence of shallow channels in the upper carbonate member, visible in outcrops, is apparent in strike sections through the seismic volumes. Landward, Bone Spring carbonate members are transitional with carbonate strata characterized by parallel reflections (Yeso Formation).

Outcrop studies (Guadalupe Mountains) of the Bone Spring and the associated Victorio Peak Fm (equivalent to the subsurface Yeso Fm) and overlying Brushy Canyon Fm (Delaware Mountain Gp) have identified a distract unit, called the Cutoff Fm, that erosionally overlies the Bone Spring and Victorio Peak formations and, in turn, is erosionally overlain by the Brushy Canyon Fm. In outcrop, the Cutoff consists of carbonate megabreccias, lime mudstones and siliceous shales. Previous studies suggested that this unit is bracketed by sequence boundaries. The Cutoff is not readily observed in the seismic data, suggesting that: a) it is not a regionally developed unit, or b) since the impedance (lithology) contrast at the base of the Cutoff is small compared to the contrast with the overlying Delaware siliciclastics, the Cutoff appears seismically to be part of the underlying Bone Spring Fm.

The sandstone members of the Bone Spring pinch out upslope. At least locally, they can be shown to be correlative with siliciclastic or dolomitic intervals (e.g., Tubb) in the shelf carbonates. As such, the Bone Spring Sandstones and shelfal equivalents together comprise the lowstand systems tracts. The slope/basin sandstones have variable thickness and it is not typically possible to identify to distinguish lowstand fan and lowstand wedge components based on seismic criteria or well data, especially where he sandstones are thin (few 10s of m). It can be shown that at least some of the sand was supplied via submarine channels that were incised into the slope during falling stage or lowstand.

This study demonstrates the significant potential that 3-D seismic data have for modifying existing depositional sequence models for carbonate-siliciclastic shelf-slope-basin transitions. The continuous coverage and ability to view arbitrary lines through the data are key. A drawback is the small size of many 3-D surveys.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah