Determination of Thermal and Cementation Histories from 40Ar/39Ar and Ion Microprobe Stable Isotope Analyses: A San Joaquin Basin Example
K. I. Mahon, T. M. Harrison, M. Grove, and O. M. Lovera
Knowledge of the temperature and cementation histories of sedimentary basins is key to appraisal of their liquid hydrocarbon potential. Understanding the thermal history permits assessment of whether source rocks have experienced conditions appropriate for petroleum formation. The mobility of hydrocarbons and their storage capacity in sandstone reservoirs are directly related to porosity changes during diagenesis. Recent advances in 40Ar/39Ar dating (stripping of Cl- correlated Arxs, Multi-Diffusion Domain model) and development of ion-microprobe techniques for precise (±0.6^pmil) µm-scale oxygen isotopic analysis provide a basis to quantitatively determine thermal and cementation histories.
Arkosic sandstones of the Stevens turbidites, San Joaquin basin, are cemented by carbonates with minor amounts of clay and quartz. Detrital K-spars from depths of 4.12 (A4) and 6.61 km (A1) in the Stevens zone at Elk Hills yield thermal histories via the MDD model. These results indicate a broadly linear temperature rise of 9±3°C/Ma over the past 10 Ma and predict current peak temperatures that are within error (±25°C) of the measured values of 200° (A1) and 150°C (A4). Previous bulk isotopic analyses of cements from Stevens sands at North Coles Levee indicate that diagenetic pore fluids were modified by the introduction of hydrocarbons and CO2 from maturing source horizons. In situ O isotopic analyses of 10 µm spots in these ceme ts confirms this heterogeneity. A model cementation history can then be calculated by linking the oxygen isotopic composition of the cements (and temperature-dependent fractionation factor) with the thermal history independently established from thermochronometry.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California