--> ABSTRACT: Primary Compositional Control on Quartz Cement and Porosity in Deeply-Buried Sandstones, Cuslana Field, Llanos Foothills, Colombia, by E. A. Warren, A. J. Pulham, and J. Naar; #91021 (2010)

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Primary Compositional Control on Quartz Cement and Porosity in Deeply-Buried Sandstones, Cuslana Field, Llanos Foothills, Colombia

WARREN, EDWARD A.,  ANDREW J. PULHAM, and JOAQUIN NAAR

Porosity in deeply-buried (>14000') sandstones of the lower Tertiary Mirador formation in the Cusiana Field, Llanos foothills, Colombia, is less than 10 p.u. However, permeability is as much as 1000 mD; which is believed to be due to the simple quartzose detrital grain composition and large grain size. The low porosity has resulted from pervasive quartz cement (10-15 percent bv). Fluid inclusion microthermometry and intergranular grain volumes indicate that the quartz cement precipitated from hot fluids (c.100 degrees C) during deep burial. Underlying formations might be expected to have similar low porosity.

Surprisingly, thick (>400') sandstones of the underlying late Cretaceous Guadalupe Group in the Cusiana Field have porosity as high as 20 p.u. These sandstones are lithic sandstones composed of quartz and a variety of phosphate grain types and are poorly cemented. Quartz cement in the form of micro-crystalline quartz is almost entirely absent. However, inter-bedded quartz arenites within the Guadalupe Group are heavily cemented by quartz cement and have similar porosity to the overlying Mirador formation.

The major control on porosity in these sandstones appears to be the presence of quartz cement and that the quartz cement is controlled by grain composition. The reasons for the lack of quartz cement in the lithic sandstones of the Guadalupe Group are uncertain but phosphate poisoning inhibiting quartz cement growth is a possibility. The presence of high porosity 'sweetspots' in deeply buried sandstones may be linked to the occurrence of phosphate beds - a primary depositional control.

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.