Download Document

K.I. Mahon, T.M. Harrison, K.D. McKeegan (1997) The thermal and cementation
histories of a sandstone petroleum reservoir, Elk Hills, California. Part 2: In situ oxygen
and carbon isotopic results. Chem. Geol., submitted.
Liquid hydrocarbons within the Elk Hills and North Coles Levee oil fields, southern San
Joaquin basin, are largely isolated within calcite cemented reservoirs comprised of late
Miocene Stevens sandstone. We undertook mm-scale ion microprobe carbon and oxygen
isotope ratio measurements, with a typical precision of ±0.5‰, on calcite cements to assess,
respectively, the source of the carbon and the temperature of cementation. Temperatures are
calculated using the temperature dependence of the experimentally calibrated calcite-water
fractionation, assuming a constant pore fluid composition. Thermal history results reported in
the companion study indicate a broadly linear increase in temperature during the early to
middle Miocene followed by an increase in heating rate between 9 to 6 Ma. By combining
the thermal history results with the calculated cementation temperatures, a model
cementation history was derived which indicates that carbonate precipitation primarily
between 5 and 4 Ma. Conventional oxygen isotopic measurements yield a more restricted
range of isotopic compositions reflecting the averaging properties of that method.
The associated carbon isotopic measurements suggest that most of the early cements were
derived from a marine carbonate source or a mixture of marine carbonate and lighter carbon
from maturing hydrocarbons. Carbonates precipitated most recently (and thus at the highest
temperatures) contain light carbon, interpreted to result from thermal decomposition of
kerogen in the interbedded shales. Based on the light carbon values (< 10‰PDB) and low
range of temperatures over which the bulk of the cement formed, the maturation of petroleum
in the interbedded shales must largely postdate cementation.