Abstract

The Pennsylvanian-age Cline Shale produces oil and associated gas from organic-rich mudstone reservoirs unconventionally trapped along the east flank of the Midland basin. Laredo Petroleum Inc. (LPI) discovered the play in July of 2010 by drilling the first Cline horizontal well in Glasscock County and, to date, has drilled and completed more than 30 horizontal wells, outnumbering the rest of industry combined. The play was initially mapped using conventional open-hole wireline and mud logs supported by single zone tests. Once production was established, Laredo collected nearly 1,000 ft. of whole core and 700 sq miles of proprietary and licensed 3-D seismic in order to systematically quantify key geomechanical and geochemical rock properties and build stochastic petrophysical models. These parameters were instrumental not only in reservoir characterization but also in building accurate well-bore stability models and designing effective stimulations.

The reservoir is a naturally fractured, organic-rich, slightly calcareous, laminated to massive argillaceous mudstone, and is generally 250-350 ft thick between 9,000-9,500ft tvd depths in Laredo’s prospective fairway. The depositional environment is considered to be a relatively anoxic distal slope to basin plain in which sedimentation occurred mostly by settling of fine sediment from suspension, with occasional siltstone laminations formed by distal turbidity flows. These mudstones are composed predominantly of detrital quartz particles (35-55%, XRD by weight) in a matrix of compacted detrital clays (30-45%, XRD by weight) intermixed with lesser amounts of organic material.

The Cline Shale is a repeatable resource play that compares favorably to other productive shale-oil formations. The source potential is excellent with an abundance of oil-prone kerogen (2-8% TOC) currently in the peak oil window of generation (0.8-1.1% Ro). Reservoir properties such as effective porosity (5-8%) and absolute matrix permeability (avg 500 nd) are comparable as well, and predictive rock models show preferential landing points for horizontal drilling. The abundance of natural fractures significantly enhances system permeability over matrix, and the formation is over-pressured (0.5-0.6 psi/ft) suggesting trap integrity. Ongoing work includes integrating rock properties from 3-D Pre-Stack (PSTM) seismic inversion, calibrating log-derived hydrocarbon pore volume estimates to actual production, and regional mapping of OIP fairways.