Sedimentologic and Stratigraphic Controls on Reservoir Sweet Spots in Wolfcamp A, Howard County, Midland Basin

Abstract

Deep-water carbonates can form significant conventional and unconventional reservoir systems, but controls on deposition and reservoir character are not still not adequately understood.

This project explores the uppermost “Wolfcamp” (“Wolfcamp A;” early Leonardian), in Howard County, Texas. Core and well log data are used to examine the Wolfcamp A as a case study to understand the sedimentologic and stratigraphic controls on ‘sweet spots’ within deep-water settings of the Midland Basin.

Core data identified eleven distinct lithofacies, grouped into seven facies assemblages. The three dominant facies assemblages are coarse-grained echinoderm and foram rudstone-packstone (CGC), fine-grained calcareous mudstone-packstone (FGMP), and siliceous mudstone-siltstone (SMS). CGC facies have sharp, locally erosive surfaces, rip-up clasts, are massive or have internal grading suggesting deposition from sediment gravity flows (SGFs). SMS and FGMP facies typically show a gradational relationship with CGC facies. The dominance of detrital quartz, lack of radiolarians, presence of shallow-water skeletal fragments, and massive or normal graded laminations suggest SMS and FGMP were deposited as distal portions of SGFs. SMS facies have the best unconventional reservoir potential, with unconventional total porosity ranging from ~6-10%, total organic carbon (TOC) ranging from 2-3.2 wt%, and sufficiently low clay content (<50%) for fracture success.

Three major genetic units identified in core and well logs can be correlated throughout the study area. The lowest unit (U1) is dominantly CGC facies, the middle unit (U2) is characterized by the highest abundance of SMS facies, and the upper unit (U3) is composed of both SMS and FGMP facies. Subunits in U1 are lobe-shaped and sourced from the intersection of the Eastern Shelf and Glasscock “nose”. U2 subunits, consisting of interbedded SMS and thin CGC intervals, form linear, channel-like bodies sourced primarily from the Eastern Shelf, to the NE of the Glasscock nose. U3 lobe-shaped subunits were sourced from the Eastern Shelf and Glasscock nose.

Ongoing studies are focused on additional details of facies distribution and subunit geometries, controls on deposition, and reservoir sweet spot locations. Initial results suggest unconventional sweet spots in medial to distal locations within U2, and conventional sweet spots in proximal positions within U1.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018