Extrusive Shale Masses: New Gulf Coast Exploration Frontier: ABSTRACT

Abstract

Many Gulf Coast masses are extrusive deposits formed by the processes of sedimentary volcanism. Sedimentary volcanic deposits have been recognized only recently in Tertiary strata of the Gulf Coast. Diagnostic evidence for this phenomenon is found at outcrops of the Catahoula Formation (middle Tertiary) in the south Texas counties of Live Oak, McMullen, Duval, and Webb. The absence of active sedimentary volcanism in the Gulf Coast and the difficulty of recognizing this phenomenon in ancient rocks are causes for a general omission of this subject from the American geologic literature; consequently, explorationists are overlooking diapiric and possibly extrusive origins for numerous Gulf Coast masses. The ultimate relation of a buried extrusive mass to adjacent and overlying beds is determined by the amount of mudflow buildup and preservation during the time of deposition of the nearby normally deposited beds. If the sum of mudflow deposition (with accompanying erosion) greatly exceeds the sedimentation of the adjacent beds, large mudflow domes and ridges may form prominent topographic features. Conversely, if the rate of sedimentation of adjacent beds equals or exceeds that of the mudflow accumulation, an ill-defined mudflow facies is formed. Most thick, extrusive bodies probably are composite masses of both rapidly and sporadically extruded mudflows interfingered with normally deposited beds. Growth of an extrusive dome is attained by a sequence of mudflows extruded from clusters of mudcones. Dips of mudflow layers increase as each succeeding layer is extruded and a domal topographic feature forms. Slopes of active mudcones are commonly 30-40°, depending on the mud viscosity; cones are known to exceed 1,500 ft. in height. Commonly, mudflows range in thickness from several inches to 50 ft. and extend as much as 2 mi. from their parent vent. Mudflow extrusions may take place simultaneously for many miles along a fault system. Active mudflow ridges 20 mi. long are known in West Pakistan. These flat-topped ridges are hundreds of feet thick and have steep sides with 40-70° slopes. Erratic rocks commonly are brought up thousands of feet stratigraphically by mudflows. Erratics up to 3 ft. in maximum dimensions are common, and rarer occurrences of blocks with 50-ft. dimensions are known. Microfossils, thousands of feet out of place, occur in many places within extrusive mudflows or masses. Diagnostic evidence of diapiric clastic rocks includes: erratic fossils, churned pellets, gas bubbles, and disrupted rock frameworks. Revised exploratory thinking is required to search successfully for and to recognize subsurface diapiric masses. Diapiric masses are formed in specific basins, along certain trends, and during favorable geologic times. Although intrusive plugs exhibit the same pronounced structures as salt plugs, buried extrusive masses are not associated generally with pronounced radial faulting, sharply upturned End_Page 2324------------------------------ beds, or other commonly recognized structural attributes of intrusive masses. Diapiric shales produce negative gravity anomalies because of low densities. Density logs show densities to be almost as low as salt. Low velocities (indicated by sonic logs) cause shale-mass structures to be mapped seismically as lows instead of highs, unless correct velocity functions are used. A common clue to subsurface diapiric masses is 0.5-ohm resistivity (IES log) caused mainly by high water content of the shale. Few correlations, if any, can be made in the diapiric mass. An abnormal microfaunal sequence is found in nearly every case, as is high-pressure gas. Because of their greater magnitude and distinguishable direction, mudflow dips within an extrusive mass can be recognized commonly by a dipmeter survey. Dips recorded within an intrusive plug or a shale sheath should be random in both magnitude and direction. Sidewall cores within a diapiric mass contain churned pellets and gas bubbles in the units and also bear disrupted sand-grain frameworks in the sandstone bodies. Sandy, water-filled, gas-churned mudflows are high-porosity, low-permeability masses that serve as barriers to hydrocarbon migration. Intrusive structures must have had a timely injection in order to trap migrating hydrocarbons whereas extrusive masses are unusual barriers because the barrier is present before or during deposition of the adjacent beds. Systematic recognition and delineation of extrusive masses in the Gulf Goast by both conventional and improved exploration methods will open new frontiers to Gulf Coast petroleum exploration. End_of_Article - Last_Page 2325------------