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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 38 (1988), Pages 9-17

Pore Types and their Distribution in Wilcox Formation, Central Texas Gulf Coast

Kung-Chou Chuang (1), Earle F. McBride (2)

ABSTRACT

Point counts were made of 141 samples (depths of 104 to 14,280 ft) from 13 cores in 6 counties to assess the abundance and distribution of various types of pores visible in thin sections impregnated with dyed epoxy. Previous HitPorosityNext Hit averages 13.8% and decreases at a rate of 0.6%/1000 ft. However, the rate of decrease of Previous HitmaximumNext Hit Previous HitporosityNext Hit values is twice the above rate. The average abundance of pore types as percent of total rock is 4.1% intergranular pores, 3.3% micropores in rock fragments, 2.2% intragranular pores in feldspar, 2.5% micropores in clay minerals, 1.2% oversize pores, 0.1% intragranular pores in igneous/volcanic rock fragments and 0.4% intragranular pores of other types. If all intergranular pores and micropores within clays (mostly authigenic) are primary, then primary pores make up an average of 46% of total Previous HitporosityNext Hit. Thus, secondary pores are more abundant than primary pores even using a very conservative approach.

Intergranular pores and oversize pores are the only pore categories that have statistically significant (r>0.4) Previous HitdepthNext Hit trends: both decrease with Previous HitdepthNext Hit. Intergranular pores have a positive correlation (r=+0.83) with oversize pores and kaolinite cement (r=+0.42). Both correlations are expected because complete dissolution of grains is most effective in sandstones with abundant intergranular pores, and the formation of kaolinite is favored in sandstones with large pores and high formation-water flux. The loss of K-feldspar at depths greater than 10,000 ft is not manifested by a comparable volume of oversize pores.

Only a few pore types show regional trends, in part because of sparse sample control. However, the map of intergranular Previous HitporosityTop shows an elongate NE trend of high values that follows the trend of growth faults in the area. This raises the question whether some intergranular pores may be secondary pores that formed by formation-water flow that was focussed along growth faults.


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