Sedimentological and Geochemical Variability of Mudrocks in Coal-Bearing Successions of the Carbondale Group (Pennsylvanian) in Southwest Indiana: A Core-Based Evaluation

Abstract

Sedimentological features observed in cores, along with detailed geochemical data, provide valuable insights into the variability of roof shales in Pennsylvanian rocks of the Midcontinent. These insights are valuable for predicting reservoir character for Pennsylvanian black shales that are potentially emerging plays for shale gas production. Additionally, these same features are valuable in predicting the ability of roof shales to serve as a tight seal for coalbed methane production. The USI 1–32 well is located south of the campus of the University of Southern Indiana in Vanderburgh County, Indiana. The total depth of the well is 780 feet, with cored intervals from: (1) 395 to 415; (2) 495 to 515; (3) 540 to 560; and (4) 655 to 675 feet. The cores sampled the following units from the Carbondale Group: (1) Springfield Coal of the Petersburg Formation; (2) Houchin Creek Coal and overlying Excello Shale of the Petersburg Formation; (3) Survant Coal of the Linton Formation; and (4) Seelyville Coal of the Staunton Formation. Roof shales of the Survant and Seelyville coals consist of light gray to green gray mudrocks with sparse siderite concretions. These mudrocks have TOC values ranging from 0.1 to 0.8 wt. % with total sulfur values ranging from 0.15 to 0.40 wt. % with little to no gamma ray response. These mudrocks are interpreted to have been deposited in continental settings as evidenced by low sulfur content and sparse vascular plant debris. In contrast, roof shales above the Springfield and Houchin Creek coals consist of laminated to bioturbated, black to gray mudrocks with disseminated pyrite and sparse phosphate nodules. These mudrocks have TOC values ranging from 0.9 to 27.4 wt. % with total sulfur values ranging from 0.2 to 4.6 wt. % with an elevated gamma ray response. Additionally, there is a gradual increase in bioturbation from an ichnofabric of 1 directly above the Springfield Coal to an ichnofabric of 4 at 1.5 feet above the coal. Discrete laminations within this interval also contain broken skeletal debris, including brachiopod and molluscan fauna. Likewise, the Excello Shale transitions from laminated black mudrock to bioturbated gray mudrock above the Houchin Creek Coal. In summary, the roof shales above the Springfield and Houchin Creek coals are interpreted to have been deposited in a marine setting, as evidenced by elevated sulfur content and the presence of marine fossils.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015