Disconformity

Key Takeaways

• The recognition of disconformities in subsurface well data relies on multiple lines of evidence because the parallel bedding planes above and below the contact prevent the angular geometric evidence that makes angular unconformities obvious in seismic data and outcrop: in well logs, a disconformity may be identified by an abrupt upward change in gamma ray (from marine shale above to clean carbonate below, or vice versa, reflecting the change in depositional environment across the hiatus), by a missing section (biostratigraphic zones present in regional correlations but absent at the well location, indicating that the unconformity truncated the section), by a paleosol or weathering profile (a zone of oxidized, calcretized, or karstified rock immediately below the contact that formed by subaerial weathering during the hiatus), by an abrupt change in diagenetic overprint (the rocks immediately below the disconformity having been affected by meteoric freshwater diagenesis during the subaerial exposure, producing specific mineral assemblages including dedolomite, calcite cement replacing gypsum, and dissolution-enhanced porosity), and by an abrupt change in fluid salinity or pressure between the section above and below the contact (a disconformity that separates overpressured Devonian carbonates from normally pressured Cretaceous carbonates reflects that the two systems were isolated from each other for the duration of the hiatus).
• Disconformities in carbonate platforms are commonly associated with karst development, in which carbonate rock below the unconformity surface has been dissolved and weathered by meteoric water (freshwater from rainfall) infiltrating the exposed platform during the subaerial exposure period: the karst features associated with disconformities range from shallow solution widening of fractures and inter-particle pores (enhancing permeability at the pore to fracture scale), to solution collapse breccias (where dissolution of carbonate created cavities that subsequently collapsed, producing brecciated zones with high void space and permeability), to large karst cave systems that were later filled with cave sediments (speleothems, clays, silts) and may be detected as anomalous amplitude objects on 3D seismic data; karst-enhanced reservoir porosity associated with disconformities is the primary exploration target in several major carbonate petroleum systems, including the Upper Ordovician karsted limestone of the Tarim Basin in western China (the Tazhong and Tabei fields), the Silurian Niagaran pinnacle reefs of Michigan Basin where Salina evaporite dissolution created secondary porosity, and the Permian Yates Formation carbonates of the Permian Basin where the pre-Triassic unconformity surface is associated with significant karst-enhanced reservoir quality.
• Disconformity surfaces in sequence stratigraphy correspond to the sequence boundary (the unconformity surface that separates one depositional sequence from the next) and are recognized as the surfaces where hiatus duration is greatest, where the greatest amount of section has been removed by erosion, and where the most dramatic changes in depositional environment occur between the rocks immediately below and those immediately above the surface; in shallow-water carbonate platforms, the disconformity sequence boundary is formed during episodes of relative sea level fall that expose the platform, with the duration of subaerial exposure controlling the thickness of the karst zone (from meters in a brief exposure event to tens of meters in a long-duration lowstand), the degree of dissolution (deeper karst in longer exposures), and the amount of section removed by erosion (most on the topographically elevated parts of the platform, least in the protected interior positions); the sequence stratigraphic position of a well relative to the disconformity (updip, on the platform margin, or basinal) determines what rock types and reservoir-seal relationships are present, guiding well location selection for exploration drilling targeting sub-disconformity carbonate reservoirs.
• Differentiation between a disconformity (true hiatus with missing section and diagenetic evidence of subaerial exposure) and a diastem (a very brief depositional pause without significant missing section or subaerial exposure, recorded only as a subtle lithological change or thin cemented layer) requires biostratigraphic data from microfossils or palynomorphs that can resolve the age of the beds above and below the contact: if the ages of the last fossil assemblage below the contact and the first fossil assemblage above differ by more than one biozone (typically representing hundreds of thousands to millions of years of geological time), a significant hiatus is confirmed; if the two assemblages are from the same biozone, the surface may be only a diastem; quantitative biostratigraphy using the constrained optimization (CONOP) method or graphic correlation to compare well sections against a global composite stratigraphic section is the most powerful method for quantifying hiatus duration at disconformities and for correlating disconformity-bounded sequences between wells in a regional basin framework.
• Economic significance of disconformities in petroleum systems extends beyond the local reservoir enhancement effects of karsting to include their role as regional migration barriers, lateral seals, and unconventional reservoir targets: as regional migration barriers, a disconformity surface that is overlain by a tight, impermeable basal transgressive lag or flooding surface can deflect migrating oil and gas laterally along the unconformity surface until it encounters a structural or stratigraphic closure, a pattern that has concentrated large volumes of oil in the sub-unconformity carbonate reservoirs of the Middle East (the pre-Cretaceous unconformity is a major migration pathway in several Saudi Arabian fields); as lateral seals, lateral facies changes along the disconformity surface (from porous karsted carbonate to tight, cemented carbonate or to a different lithology) can form the lateral seal of a stratigraphic trap; as unconventional reservoir targets, sub-disconformity brecciated and fractured zones may be targets for geothermal energy or for low-permeability hydrocarbon production in regions with abundant structurally complex unconformity plays.