Stratigraphic analyses of a variety of mudstone-dominated units demonstrate that the best production from source-rock plays i.e., the reservoir is also the source rock, is usually from thermally mature, pelagic-rich strata that can be assigned to the transgressive systems tract (TST) and a condensed section (CS). They are often referred to as “black shales” although clay minerals can form 20% or less of the rock. A high Total Organic Carbon (TOC) content of typically Type I/II kerogens made these strata potential source rocks, and biogenic silica and/or carbonate produced “brittle” rock that is suitable for hydraulic fracture stimulations. Where deposition from suspension dominated, relatively thin (

The lithologic and stratigraphic characteristics of the strata differ considerably from progradational mudstones of the highstand systems tract (HST) and the lowstand systems tract (LST). The latter are characterized by higher clay contents and more detrital silicate/carbonate as silt or coarser grain sizes. The clay content generally makes these shales less brittle than the TST/CS units.

These HST/LST mudstones have lower TOC contents than the TST/CS mudstones, and the organic matter is relatively enriched in terrestrial Type III kerogen constituents. These strata tend to be more heterolithic than the TST/CS mudstones, and may be arranged into submarine failure complexes, prodelta lobes or other stratigraphic features that can show rapid lateral variations in lithology and thickness. These characteristics generally make these HST strata poorer source rocks and more complex to development using horizontal wells than TST/CS mudstones.

Although the descriptions and interpretations presented above have general applicability i.e. they are the “simple case”, these concepts commonly need to be modified to account for the wide variability in depositional setting, depositional history, and geologic age, represented by gas-productive mudstones.

Figure 1. Chonchoidal fracture, a brittle response, in organic-rich mudstones. The depositional processes responsible for making these rocks organic rich are also responsible for making them brittle.