Porosity Development in Deeply Buried Sandstones, San Joaquin Basin, California

HORTON, ROBERT A., JR, and MCCULLOUGH, PATRICK T., Department of Geology, California State University, Bakersfield CA

The discovery of natural gas in what is thought to be commercial quantities has spurred interest in porosity development in deeply buried sandstones in the San Joaquin basin, but little published literature exists. This study documents porosity development in Oligocene-Miocene arkosic sands from six deep wells (>13,000 ft) in the central basin.
Sandstones in these wells are highly indurated to the point that a hammer is generally necessary to break the core. Cements include calcite, dolomite, and clay minerals, but these are generally minor constituents in clean sands. Induration is the result of compaction, which has resulted in interlocking and sutured contacts between silicate grains. Porosity in these wells is highly variable but frequently ranges between 10-15% with values up to 20%. The bulk of this porosity is intergranular, but secondary, in nature and is not evenly distributed, even on the scale of a thin section. Rather, zones of very high porosity follow fractures and other ëchannel waysí within the sands. These ëchannel ways,í which are typically a few tenths of a millimeter wide, may follow bedding or cut across it. They served as conduits for diagenetic fluids (modified sea water and possible hydrocarbons) which were highly reactive with silicate grains. Secondary porosity, in the form of intragranular, oversize, elongate, and moldic pores, may exceed 50% immediately adjacent to these ëchannel waysí but decreases to almost zero over very short distances at the edges of the zones. The violent and sustained nature of a wildcat-well blowout near Lost Hills last year suggests that the porosity ëchannel waysí form an interconnected permeable network.
 

AAPG/SEPM Pacific Section Meeting, Long Beach, June, 2000

Porosity Development in Deeply Buried Oligocene-Lower Miocene Sandstones, Central San Joaquin Basin, California

HORTON, ROBERT A., JR, and MCCULLOUGH, PATRICK T., Department of Geology, California State University, Bakersfield CA

Oligocene-Lower Miocene sandstones (including the Temblor Formation) historically have been important petroleum reservoirs along the western margin of the San Joaquin basin. These strata also have been productive in the deeper portions of the central basin, but lack of obvious structural targets has limited exploration efforts in this area. However, recent developments in the Lost Hills area have rekindled interest in deeply buried Temblor sandstones as potential exploration targets. One big question in these efforts is the nature of porosity and permeability in the deeply buried sediments. This study documents porosity development in Oligocene-Miocene arkosic sands from five deep wells (11,000 to 15,000 ft) in the central basin.

Sandstones in these wells are highly indurated to the point that a hammer is generally necessary to break the core. Cements include calcite, dolomite, and clay minerals, but these are generally minor constituents in clean sands. Induration is the result of compaction, which has resulted in interlocking and sutured contacts between silicate grains. Porosity in these wells is highly variable but frequently ranges between 10-15% with values up to 20%. The bulk of this porosity is intergranular, but secondary, in nature and is not evenly distributed, even on the scale of a thin section. Rather, zones of very high porosity follow fractures and other ëchannel waysí within the sands. These ëchannel ways,í which are typically a few tenths of to a few millimeters wide, may follow bedding or cut across it. They served as conduits for diagenetic fluids (modified sea water and possible hydrocarbons) which were highly reactive with silicate grains. Secondary porosity, in the form of intragranular, oversize, elongate, and moldic pores, may exceed 50% immediately adjacent to these ëchannel waysí but decreases to almost zero over very short distances at the edges of the zones. Mass balance calculations indicate significant movement of dissolved materials. This suggests that the porosity "channel ways" form an interconnected permeable network.