Skeletal Porosity Research Articles

Petrography of a Reef Complex in Lower Cretaceous James Limestone, Fairway Field, Texas: ABSTRACT

The James Limestone at Fairway field and vicinity is an elongate, northwest-trending, atoll-like reef complex about 9 miles wide, 36 miles long, and 200 feet in maximum thickness. Along the northeast, southeast, and southwest margins of the reef complex, reef calcarenite interfingers with and grades laterally into clayey calcisiltite and calcareous shale of open-marine origin. Toward the northwest, reef calcarenite passes rather abruptly into oolitic calcarenite. The development of the complex is characterized by a main reef phase followed by a lagoonal phase. During the main reef phase, two parallel-trending contemporaneous reef cores were constructed on a muddy, calcareous foundation unit. Both cores are marked by a distinct vertical zonation of bio-constructed limestones which from top to bottom include: (a) rudistid limestone, (b) Chondrodonta (pelecypod) limestone, and (c) algal-spongiomorph limestone. As the reef cores grew, considerable reef-derived calcarenite accumulated peripherally to the reef cores and far exceeded the reef cores in areal extent. Near the end of the main reef phase, a shallow depression created between the reef cores became the site of a lagoon in which was deposited an interbedded sequence of foraminiferal-pelletal calcarenite, algal nodule-bearing calcilutite and biostromes of rudistid and Chondrodonta limestone. With the exception of additional minor growth of reef cores, lagoonal limestone deposition prevailed through a limited area and climaxed the development of the reef complex. The sequence of diagenetic processes operative on the reef complex is summarized as follows: 1. Skeletal breakdown and grain diminution caused by boring organisms at the time of reef growth. 2. Post-depositional leaching of the reef complex, resulting in the development of skeletal modic porosity and sparry calcite cementation of reef calcarenite. 3. Impregnation of certain parts of the reef complex by a hard, brittle, bituminous (?) substance causing local reduction of porosity and permeability. End_of_Article - Last_Page 452------------