Early-Formed Fractures: Precambrian-Like ‘Molar Tooth’ Mudcracks in Phanerozoic Unconventional Mudstone Reservoirs

Abstract

Fractures, both stimulated and natural, are critical to producing unconventional source-rock reservoirs. Open natural fractures commonly enhance production. However, where mineralized these fractures might still have higher permeability than the matrix and also can act as planes of weakness during stimulation. This presentation describes early-formed, mineralized fractures (veins) common to several unconventional reservoirs. These crumpled veins are differentially compacted in the muddy host-rock, indicating an origin during shallow burial. Veins are typically ribbon-like, and can be isolated or in a ‘swarm’. They commonly have mm- to cm-scale kinematic apertures, mm- to at least cm-scale lengths, and mm- to dm-scale heights. Veins can be filled with barite, calcite, dolomite, silica, euhedral to anhedral pyrite, and fragments of the host sediment. These mineralized fractures are akin to ‘molar tooth’ structures (MTS)—synsedimentary veins thought to be restricted to Precambrian shallow subtidal mudstones. MTS have similarly sized, crumpled, ribbon-like morphologies, but can also occur as spherical blebs. They are commonly differentially compacted with host sediment and reworked into lags at the base of storm beds, indicating an origin shallow in the sediment pile. MTS are distinguished by their distinctive fill: uniform carbonate microspar, with minor amounts of pyrite or host-sediment. Many origins have been proposed for MTS, including synaeresis, earthquake or storm wave loading, and gas expansion. Most MTS are best explained as mineralized bubbles and expansion cracks from the decay of organic matter in cohesive, porous muds. A similar origin is likely for veins in Phanerozoic unconventional reservoirs, known to be porous, cohesive, and rich in organic carbon. The restriction of MTS to Precambrian rocks has previously been attributed to sediment rheology and pore water geochemistry: MTS required cohesive muds, the absence of bioturbation, and rapid cementation from modified seawater. The occurrence of MTS-like veins in Phanerozoic source rocks extends the range for these unusual sedimentary structures outside of the Precambrian—indicating that similar sediment rheology existed locally. It indicates also that rapid cementation was possible in Phanerozoic sediments, though the cements morphologies differ. Finally, such veins may be an important indicator for primary organic-richness in ancient muddy sediments.

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