The oil fields of the Ventura Basin are very mature with many wells drilled before the advent of modern logging tools. Despite the years of development, there remain opportunities for cost-effective production, given the great thickness of the productive intervals and the dynamic-nature of the depositional and tectonic environment that created them.
Cased-reservoir analysis in existing wells is an efficient tool to target productive intervals. The well-logs and analysis examples utilize pulsed-neutron technologies. The cased-hole neutron and density porosities from the pulsed-neutron system provide the porosity information and describe gas-bearing intervals. Water flooding and gas injection has changed many of the original fluid properties. In the log analysis examples, Carbon/Oxygen logging describes the water saturations, and Sigma logging helps in the discrimination of the sands from the mudstones.
Introduction
Pulsed-neutron technologies offer engineers and geoscientists analysis tools for describing reservoirs in existing cased wells. Two log analysis examples show the applications of these technologies to the fields of the Ventura Basin.
The low salinity of the reservoir water and secondary water flooding require Carbon/Oxygen logging to measure the water saturation. In the log examples the water saturation is measured using CATO, a normalized ratio of the oxygen channel to the total counts.1 The lengthy productive intervals in this area (e.g. 1000's of feet) require some practical limits be set on sampling times. In the log analysis examples, a single six-foot per minute pass acquired the entire data set, with a single repeat pass to check tool performance and repeatability. Repeats over zones of interest can be merged to minimize the Carbon/Oxygen measurement statistics.
Several recent publications describe the pulsed-neutron porosities2 and the development of a density porosity3,4 based on scattering of inelastic gamma rays. The cased-hole porosities (neutron porosity referenced as RPHI, density porosity referenced as IPHI) and the clay volume are used to derive the effective porosity PHIE. The effective porosity is the total porosity (from the cross-plot) minus the clay bound porosity; thus it can be considered the porosity available for fluid movement.
Sigma, the capture cross-section, is also recorded. Although the contrast of oil to water is limited by the low water salinity, it is still useful for clay volume and gas saturation.
Geological setting
Figure 1 is a map of the oil fields and major structural features in the western portion of the Basin.
The ancient Ventura Basin was a trough-like deep-water basin, with the long axis in the current east-west direction5. This depocenter entrapped thousands of feet of sediment as the basin floor subsided during Pliocene time. Facies analysis suggest that the sands are turbidites deposited as elongated lenses, spread along the axis of the basin by westerly currents. The source of the detrital material was from uplifted Paleocene and Miocene sediments north of the Red Mountain and San Cayetano faults. The sands in the analysis examples are arkosic with angular feldspar and quartz grains. Organic detritus, such as coal and wood fragments, have been noted. Intervals of mud and silt debris flows and deposition of marine shales overlap the turbidite sands.
Folding and faulting of these Pliocene sediments have created traps for the accumulation of hydrocarbons. Common reservoir compartments are anticlines and blocks truncated by faults. On a smaller scale, the reservoirs have stratgraphic compartments from features such as clay and calcite seals. Further, the grading and pinching-out of the lenticular sand bodies create depositional reservoir compartments.