Abstract: Delineation of a Regional Meander Belt Using the Formation Microscanner and Other Wireline Log Data, Upper Devonian 3rd Bradford Sand, Centre, Clinton, and Lycoming Counties, Pennsylvania
John F. Humphrey, Paul A. Babasick, Robert J. Davis
Regional studies over a 600 mi2 area in north central Pennsylvania using conventional wireline log data plus the Formation Microscanner (FMS) have revealed the existence of an extensive meander belt system in the Upper Devonian 3rd Bradford sand. Many fields have been discovered in the 3rd Bradford sand within the meander belt, most being 10 wells in size on 40 ac spacing. The shallow depth of this zone (2500-3800 ft) and sizable reserves make it an attractive target using currently available technology.
Using a database of over 700 wells, we studied the gamma-ray character of the Bradford sand for characteristics that would define the depositional sequence of the unit, and determined that the Bradford was a regionally extensive meander belt with a width of approximately 6000-13,000 ft. The vertical sequence seen on the FMS in the thicker part of the point bar shows a massive unit at the base of the sand exhibiting large-scale cross-bedding, which is clearly visible on the FMS images. Scour at the base of the sand is also visible on the images. Overlying the large-scale cross-bedding is a sequence composed of convolute and parallel laminations. In many instances, multiple bars have been stacked on top of one another, this being confirmed by the data collected from the FMS. Total thick ess of the bar sequence is generally 50-80 ft, with the best porosity development at the base of the unit. Other facies within the meander belt (abandoned channel fill, overbank deposits) can be readily identified by gamma-ray log signature. Data from rotary sidewall cores indicate the sediment source to be close (angular grains, mineralogically immature sublitharenites-metamorphic source).
Due to the sinuosity of meandering channels, FMS data is very important in site selection. Many important indicators (direction of flow, location of the thalweg, basal contacts) can be derived from the data to aid in the selection of step-outs and development wells. Reduction of the risk inherent in this type of reservoir increases the economic viability of the play.
AAPG Search and Discovery Article #90984©1994 AAPG Annual Convention, East Lansing, Michigan, September 18-20, 1994