Geochronology

Key Takeaways

• U-Pb zircon geochronology is the most precise and most widely applied geochronological method in petroleum source rock and reservoir provenance studies, using the decay of uranium isotopes (U-238 decaying to Pb-206 with a half-life of 4.47 billion years, and U-235 decaying to Pb-207 with a half-life of 704 million years) within the crystal lattice of zircon (ZrSiO4) grains that exclude lead from the crystal at the time of crystallization but retain the radiogenically produced lead afterward; the two independent decay systems (U-238 to Pb-206 and U-235 to Pb-207) provide a concordance check — if the two ages agree, the zircon has remained a closed system and the measured age is the true crystallization age; in detrital zircon studies for reservoir provenance analysis, individual zircon grains are dated by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), measuring U and Pb isotopes in a 25-50 micrometer spot burned into each grain, allowing age distributions from hundreds of grains per sample to be generated in a single day; these detrital zircon age distributions are compared between wells and between basins to trace the routing of sediment from source regions to reservoir, which is critical for predicting reservoir quality (sand maturity, grain size distribution) in frontier exploration areas where core control is sparse.
• Fission track thermochronology, using the spontaneous fission of U-238 in apatite and zircon minerals to produce damage trails (fission tracks) whose length and density record the thermal history of the mineral since it cooled below the track annealing temperature, is the principal geochronological tool for reconstructing the burial and uplift history of petroleum source rocks: apatite fission tracks anneal (shorten and eventually disappear) at temperatures above approximately 100-120 degrees Celsius (the apatite fission track closure temperature, equivalent to burial depths of 3,000-4,000 meters in typical geothermal gradients), and accumulate progressively as the mineral cools below this temperature during uplift or as the geothermal gradient decreases; the mean track length (typically measured on 50-100 tracks per sample) and the track density (related to the time elapsed since cooling through the closure temperature) together constrain the time-temperature path of the sample, allowing the geologist to determine when the source rock was at maximum burial temperature (the peak maturation phase for oil generation) and when it was uplifted and cooled (stopping generation); in the Southern Rocky Mountain foreland basin, fission track data from wells and outcrops has been used to constrain the amount of exhumed overburden since the Laramide uplift, resolving a fundamental uncertainty in the burial history that affects the calculation of source rock maturity for petroleum exploration in the structurally complex Rocky Mountain basins.
• Re-Os geochronology of organic-rich shales directly dates petroleum source rock deposition by using the radioactive decay of rhenium-187 (Re-187) to osmium-187 (Os-187, half-life of 41.6 billion years) in the organic matter and sulfide minerals of black shales; Re and Os are enriched in organic-rich sediments because they are highly compatible with organic matter and with pyrite (the iron sulfide mineral that forms in anoxic marine and lacustrine sediments where organic preservation is highest), allowing isochron dating of the source rock deposition event from multiple coeval samples with different Re/Os ratios that define an isochron line whose slope gives the age; Re-Os dating has been applied to the Devonian black shales of eastern North America, the Kimmeridgian source shales of the North Sea, the Bazhenov Formation of Western Siberia, and other world-class source rocks to provide age constraints that help correlate source rocks with their oils (by comparing the isotopic composition of Re and Os in the oil with that of the candidate source rock) and to constrain the timing of source rock deposition relative to basin evolution events; this oil-source correlation application of Re-Os is particularly powerful in basins where multiple source rocks of different ages could have contributed to the petroleum accumulation, and where traditional biomarker correlation (which relies on biological markers of the source organic matter) is ambiguous.
• Thermochronological modeling of petroleum generation combines geochronological constraints on the burial history of a basin with thermal maturation models (Easy%Ro, PetroMod, BasinMod) to predict when a source rock entered and exited the oil window, providing critical input to charge risk assessment in exploration: the burial history model (derived from stratigraphic data on sediment thickness and age, geochronological constraints on unconformity age and magnitude of erosion, and paleo-heat flow reconstructions) is the primary input to 1D and 2D basin modeling; thermochronological data (fission track ages and track length distributions from wells and outcrops, vitrinite reflectance profiles from well logs) provide direct constraints on the integrated time-temperature history of the section that calibrate the burial model and the heat flow history; the calibrated model then predicts the transformation ratio of the source rock (the fraction of original petroleum generation potential that has been realized), the timing of peak oil generation (which must predate trap formation to be retained), and the depth range of the oil and gas window in the current basin configuration; uncalibrated basin models that rely on assumed rather than measured thermal histories carry large uncertainties in predicted oil generation timing that propagate into exploration risk assessments.
• Geochronological age dating of volcanic intrusions, hydrothermal events, and diagenetic minerals uses methods including Ar-Ar dating of volcanic glass and feldspar (recording the time of cooling below the argon retention temperature after intrusion), K-Ar dating of diagenetic clay minerals including illite (which grows at the expense of kaolinite during burial and retains the time of illite crystallization that corresponds to maximum burial temperature), and U-Pb dating of diagenetic calcite, dolomite, and fluorite cements (providing the age of cementation that records the timing of fluid flow through the reservoir): illite K-Ar dating has been applied to North Sea Rotliegend sandstone reservoirs to date the timing of gas emplacement, since the growth of diagenetic illite is suppressed in reservoir intervals saturated with hydrocarbons (because the elevated salinity of the connate water associated with the gas inhibits illite growth), allowing the age of the gas-water contact to be determined from the contrast in illite abundance and age between the gas-saturated and water-saturated portions of the reservoir; this timing constraint is one of the few direct methods for determining when a petroleum accumulation was charged, essential for understanding the relationship between hydrocarbon emplacement and trap formation in complex structural settings.