The diagenetic destruction of porosity in plutoniclastic petrofacies: The Miocene Diligencia and Eocene Maniobra formations, Orocopia Mountains, southern California, USA
The Miocene Diligencia Formation of southern California was deposited in an alluvial/fluvial depositional system developed within the Diligencia basin. This basin formed as an extensional half-graben during latest Oligocene–Early Miocene crustal extension. This continental half-graben was superposed on an Eocene fault-controlled submarine canyon, in which the Maniobra Formation was deposited.
The arkosic composition of the sandstone framework would predict a high-quality reservoir with preserved primary porosity or at least important intergranular volume (IGV). However, inspection of sandstone samples indicates that diagenesis was intense, as several diagenetic processes drastically reduced the reservoir quality. Dominant diagenetic processes were compaction and cementation. Mechanical compaction, expressed as intense deformation of ductile grains and chemical compaction in the form of pressure solution, reduced the IGV of sandstone at the bases of the Maniobra and Diligencia formations to low levels (10–20%). In the upper parts of both units, early diagenetic cements partially inhibited compaction, maintaining IGV values close to 35%.
Several mineral phases constitute cements responsible for occlusion of primary porosity. Many of these phases also replaced framework components and early cements. The more important cements are: (1) quartz and K-feldspar, which appear mainly as grain overgrowths, although quartz mosaics have been observed at the base of the Maniobra Formation; (2) carbonates, such as ankerite, dolomite and calcite in diverse textures and with significant occluding character; (3) phyllosilicates, such as kaolin, that developed mainly in Diligencia sandstone as early pore fillings, and locally as illite coats around detrital grains; and (4) other mineral phases, such as Fe-oxides and fluorite, which occur exclusively at the base of the Maniobra Formation, exhibiting aggressive textures against framework grains and older cements.
The chronology of diagenetic processes includes marine early diagenesis (eodiagenesis) for Maniobra sandstone, characterized by K-feldspar, ankerite and dolomite cements. Continental early diagenesis is identified in Diligencia sandstone, manifested by the presence of smectite (replaced by illite), kaolin and Fe-oxide cements. Mesodiagenetic processes are similar in both formations. In addition, hydrothermal phases (fluorite and quartz mosaics) are identified at the base of the Maniobra Formation. Diagenetic mineral associations suggest palaeotemperatures above 130 °C.
The geotectonic scenario in which diagenesis occurred explains these post-depositional processes. During latest Oligocene–Early Miocene crustal extension, the Diligencia basin developed in an area of high heat flow, as expressed in the eruption of interbedded basaltic–andesitic lavas. High heat flow favoured compaction and cementation, accelerating these diagenetic processes during a relative short time interval. Hydrothermal fluxes produced mineral phases that contributed to the destruction of a potentially good reservoir by intense diagenesis.