3-D SEISMIC EVIDENCE OF THE EFFECTS OF CARBONATE KARST COLLAPSE ON OVERLYING CLASTIC STRATIGRAPHY AND RESERVOIR COMPARTMENTALIZATION

A multidisciplinary team, composed of stratigraphers, petrophysicists, reservoir engineers, and geophysicists, studied a portion of Boonsvil le gas field in the Fort Worth Basin of north-central Texas to determi ne how modern geophysical, geological, and engineering techniques can be combined to understand the mechanisms by which fluvio-deltaic depos itional processes create reservoir compartmentalization in a low- to m oderate-accommodation basin. An extensive database involving well logs : cores, production, and pressure data from more than 200 wells, 26 mi (2) (67 km(2)) of 3-D seismic data, vertical seismic profiles (VSPs), and checkshots was assembled to support this investigation. We found t he mast Important geologic influence on stratigraphy and reservoir com partmentalization in this basin to be the existence of numerous karst collapse chimneys over the 26-mi(2) (67 km(2)) area covered by the 3-D seismic grid, These near-vertical karst collapses originated in, or n ear, the deep Ordovician-age Ellenburger carbonate section and created vertical chimneys extending as high as 2500 fl (610 m) above their po int of origin causing significant disruptions in the overlying elastic strata. These karst disruptions lend to be circular in map view, havi ng diameters ranging from approximately 500 ft (150 m) to as much as 3 000 ft (915 m) in some cases. Within our study area, these karat featu res were spaced 2000 ft (610 m) to 6000 ft (1830 m) apart, on average. The tallest karst collapse zones reached into the Middle Pennsylvania n Strawn section, which is some 2500 ft (760 m) above the Ellenburger carbonate where the karst generation began. We used 3-D seismic imagin g to show how these karst features affected the strata above the Ellen burger and how they have created a well-documented reservoir compartme nt in the Upper Caddo, an upper Atoka valley-fill sandstone that typic ally occurs 2000 ft (610 m) above the Ellenburger. By correlating thes e 3-D seismic images with outcrops of Ellenburger karat collapses, we document that the physical dimensions (height, diameter, cross-section al area) of the seismic disruptions observed in the 3-D data equate to the karst dimensions seen in outcrops. We also document that this Ell enburger carbonate dissolution phenomenon extends over at least 500 mi (800 km), and by inference we suggest karst models like we describe h ere may occur in any basin that has a deep, relatively thick section o f Paleozoic carbonates that underlie major unconformities.