Metagenesis

Key Takeaways

• Vitrinite reflectance as the thermal maturity indicator for metagenesis provides the primary stratigraphic index used by geochemists to determine whether a source rock has entered, is in, or has passed through the metagenesis window, based on the empirical relationship between the reflectance of vitrinite macerals (fragments of woody plant tissue preserved in fine-grained sedimentary rocks) and the maximum paleotemperature experienced by the rock: vitrinite grains are particularly useful maturity indicators because their reflectance increases monotonically with heating (they do not re-equilibrate at lower temperatures after heating, preserving a record of the maximum thermal stress), because they are ubiquitous in most fine-grained clastic and carbonate source rock sequences as vitrinite-rich humic coals and coalified plant debris, and because they can be measured precisely on polished rock section under a reflected-light microscope; the reflectance scale used for maturity classification defines immature source rocks at Ro below 0.5% (diagenesis zone, no oil generation), early mature (oil window onset) at 0.5-0.7% Ro, peak oil generation at 0.8-1.2% Ro, late mature (wet gas and condensate) at 1.2-2.0% Ro, and metagenesis (dry gas cracking to methane and graphitization) at Ro above 2.0%; source rocks with Ro above 3.5-4.0% have completely converted their organic matter to graphite and are geologically "dead" from a petroleum generation perspective, with any residual carbon inert and unreactive under normal burial and temperature conditions.
• Dry gas generation during metagenesis produces thermogenic methane by the cracking of the C-C bonds in heavy kerogen molecules and the C-C bonds in previously generated oil and wet gas hydrocarbons that have been retained in the source rock or in tight associated reservoirs: the methane generated in the metagenesis stage is isotopically heavier in carbon-13 than the methane generated in early catagenesis (the biogenic and early thermogenic methane has delta-13C values more negative than -50 per mil, while late thermogenic dry gas from metagenesis has delta-13C values of -30 to -20 per mil), providing a geochemical fingerprint that distinguishes deep, high-maturity dry gas from shallower thermogenic or biogenic gas; the very dry character of metagenesis-generated gas (methane percentage above 98%, with negligible ethane, propane, or heavier components) further distinguishes it from early-catagenesis wet gas (which has significant C2-C5 components) and from gas produced by biodegradation of previously generated oil (which has distinctive molecular and isotopic signatures); gas fields sourced from metagenesis-grade source rocks are found in deeply buried basins where subsidence has carried sedimentary sequences to depths of 5,000-8,000 meters, such as the Haynesville Shale in Louisiana, the deep Tuscaloosa Marine Shale, and the deep Rotliegend gas fields of the North Sea and Central European basins.
• Exploration implications of metagenesis-level source rocks affect both the assessment of petroleum generation risk in deep basins and the identification of residual gas potential in overmature shale gas plays: from a conventional exploration standpoint, a source rock that has entered metagenesis has finished generating petroleum and any oil or wet gas previously generated has either migrated out of the kitchen into reservoirs at shallower burial depths or has itself been cracked to dry gas and pyrobitumen (the solid residue of cracked oil); the conventional petroleum system associated with a metagenesis-grade source rock kitchen therefore has its oil and wet gas accumulations not at the current location of the kitchen but up-dip and up-structure from the overmature source, where migrated oil and gas encountered structural or stratigraphic traps at lower thermal maturity levels; for unconventional shale gas exploration, the metagenesis window is actually the target zone in ultra-dry gas shales where the goal is to produce the in-situ thermogenic methane generated by the final cracking of residual organic matter, and these plays require hydraulic fracturing to achieve commercial production rates from the tight, organic-rich matrix; the Barnett Shale, Fayetteville Shale, and Marcellus Shale plays in the eastern US include portions that are in the late catagenesis to early metagenesis window and produce ultra-dry gas from the in-situ thermogenic generation.
• Pressure-temperature-time (P-T-t) path reconstruction for source rocks that have entered metagenesis uses a combination of vitrinite reflectance, apatite fission track thermochronology, and fluid inclusion microthermometry to reconstruct the burial and heating history that brought the source rock to metagenesis temperatures, enabling geologists to determine when generation occurred and what migration distances and pathways are consistent with the observed accumulation geometry: apatite fission track analysis provides the cooling history of the rock (constraining when the rock was at specific temperatures on the way up through the temperature-depth profile after maximum burial), complementing the vitrinite reflectance measurement which gives the maximum paleotemperature but not the timing; fluid inclusions trapped in diagenetic cements in associated reservoir rocks preserve samples of the paleofluids present during cementation, and the homogenization temperatures of these inclusions constrain the temperatures at which the fluids existed, providing independent constraints on the burial depth and temperature at the time of hydrocarbon charge; the integration of these thermal indicators in a 1D or 2D basin model calibrated to the observed maturity profile provides the reconstruction of the P-T-t path that is the foundation of the petroleum system model and the charge risk assessment for exploration prospects in the basin.
• Graphitization as the ultimate endpoint of metagenesis represents the complete conversion of kerogen from a complex macromolecular organic polymer to a crystalline inorganic carbon solid (graphite), a process that begins during metagenesis above approximately 150-200 degrees Celsius and is complete above approximately 300 degrees Celsius at geological timescales: the graphitization process can be monitored using the Raman spectroscopy peak ratio of the D-band (disorder band at approximately 1350 cm-1) to the G-band (graphite band at approximately 1580 cm-1), with highly disordered organic matter showing a large D/G ratio and well-crystallized graphite showing a near-zero D/G ratio; Raman spectroscopy of dispersed organic matter in metasedimentary rocks (schists, slates, phyllites) can extend the thermal maturity scale beyond the range where vitrinite reflectance is reliably measurable (above approximately 4-5% Ro, where the vitrinite grains are so reflective that measurement is difficult), providing maturity information for strongly metamorphosed source rock sequences in orogenic belts; the presence of graphitized organic matter in a source rock horizon definitively indicates that all petroleum generation potential has been exhausted and that any petroleum system analysis in the basin must account for the loss of the source rock contribution at depth, focusing exploration on reservoir intervals that received migrated charge from the source rock before graphitization was complete during the burial history.