ABSTRACT: The Burns Sand, La Honda Basin, California: Reassessment of its Lithology and its Hydrocarbon Reservoir Potential
EL-SABBAGH, DALLILAH, Dames & Moore, San Francisco, CA
The Burns sand of the South La Honda oil field, Santa Cruz Mountains, California, is an oil-bearing, early to middle Miocene tuffaceous limestone. The unit consists mainly of reworked shallow-water calcareous sediments, with as much as 65% volcaniclastic debris, deposited on the shelf edge and upper flanks of a submarine topographic high in a siliciclastic-dominated basin.
Volcanic activity within the La Honda Basin created a local bank-top shallow-water environment where calcareous organisms flourished and produced large volumes of carbonate sediment. Chemically unstable ash and finely crystalline basalt from local submarine volcanic activity were introduced to the bank-top setting and mixed with the bioclasts during reworking and redeposition along the bank edge and flanks.
The uncommon mixture of calcareous and volcaniclastic sediments served to influence the sequence of alterations during burial and diagenesis. Alteration of the highly unstable tuff to void-filling clays and zeolites in the more tuffaceous-rich layers of the unit resulted in a decrease of porosity and permeability. In the carbonate-rich oil-bearing layers, primary and secondary porosity remained high, enabling those layers to become hydrocarbon repositories. The presence of calcium carbonate ions in the pore waters appears to have influenced the sequence of zeolite formation, enabling the formation of sedimentary analcime at shallower burial depths and lower burial temperatures than have previously been observed. The decrease in permeability coupled with the released of magnesium and i on from chemically weathered basalt clasts may have been conducive to the formation of increasingly more iron-rich calcite and dolomite cements in stratigraphically deeper sections of the unit.