High Resistivity, High Porosity (Apparently) Monterey Formation: What Is Its Lithology And How Is It Recognized On Logs?
Below the Ct/quartz transition in Monterey-type siliceous rocks, there is a facies characterized by high (can be >100 ohm-m), usually spiky resistivity, and moderate to high log porosity for the depth. Apparent log porosity can be >20% at 8000-9000', and >10% below 13,000'. GR logs are variable, and sometimes there is SP development. Mudlogs often describe chert and porcelanite with low clay/clastic content, dark brown to black colors, and sometimes mention organic matter. In many wells, oil shows are low and gas is moderate through this facies. Conventional interpretation of the logs shows significant amounts of hydrocarbons are present, but production tests often do not meet expectations. This facies is present over large areas in the deep San Joaquin basin, and has been observed in scattered West Side wells, indicating it is probably widely distributed there also. After study of log responses, mudlogs, well performance, and some RockEval data, it appears that a large fraction of the porosity in this Monterey facies is either filled with kerogen or has been isolated by diagenetic processes. Logs “see” kerogen and isolated pores as if they are normal porosity. Resistivity logs only respond to the fluids contained in the interconnected pore system, and resistivity increases as porosity decreases. A rock that appears to have a low Sw at 15% log porosity actually is wet at 5% real interconnected porosity. Therefore, in evaluating highly siliceous and organic-rich Monterey-type rocks, it is necessary to consider whether the presence of isolated porosity is affecting the log responses, and to incorporate mudlog, cuttings, cores, and other data into the analysis. Monterey rocks with significant isolated porosity can be productive if they are situated in a favorable structural and stratigraphic position. However, for proper reservoir characterization it is necessary to recognize the real rock properties of this facies and plan the development accordingly. In conclusion, when this facies is present, decisions about whether the rock is pay or not depend on careful analysis of all the available data.
AAPG Datapages/Search and Discovery © 2014 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Bakersfield, California, April 27-30, 2014