ABSTRACT

During the Late Cretaceous, the Rio Escondido Basin was filled with deposits of lobate deltas that prograded eastward. Three distinct depositional environments can be recognized in the subsurface: delta plain (Olmos Formation), delta front (San Miguel Formation), and prodelta (Upson Formation).

The sandstone of the Rio Escondido Basin is predominantly feldspathic litharenite. Major framework grains are quartz (23%), plagioclase feldspar (35%), and volcanic and sedimentary rock fragments (42%). Most of the matrix was formed by compaction of sedimentary and volcanic rock fragments during early burial. Porosity was reduced to 19% by compaction before cementation. Chlorite, the earliest cement, formed thin rims around framework grains and replaced feldspar rock fragments, and matrix. Minor thin quartz overgrowths precipitated next. Extensive calcite cement occluded most remaining porosity by filling intergranular pores and replacing framework grains (chiefly feldspar). Subsequently, widespread dissolution of calcite created 5-10% secondary porosity. Kaolinite precipitated next as a pore-filling cement and today is the chief occluder of secondary porosity. Ferroan dolomite replaced most remaining calcite; only remnant calcite is present in the sandstone today. Late-stage cements include local authigenic siderite and pyrite. A final dissolution event locally removed remaining calcite cement or framework grains (feldspar).

Essentially all porosity present in the sandstone today is of secondary and not primary origin. Macroporosity in the sandstone is 5.0% and permeability is 2.6 md. Kaolinite cement, volcanic rock fragments, and matrix contain an estimated 6.5% microporosity. Porosity distribution is patchy and cannot be correlated with depositional facies. Porosity and permeability values do not indicate good reservoir quality in the northern Rio Escondido Basin sandstone.