Gerald E. Drake Jr., Jeff L. Daniels, Andrea S. Steinle, and Stacy P. Williams

Union Pacific Resources Company

ABSTRACT: Horizontal Drilling in the Cretaceous Austin Chalk Formation-Case Studies

The Late Cretaceous Austin Chalk formation is present in the subsurface paralleling the US Gulf Coast from Mexico to Florida and produces from numerous fields in Texas and Louisiana along a trend 500 km in length and 50 km in width. There have been multiple stages of exploration and development in the Austin Chalk since the first wells were drilled in the 1920’s. Conventional drilling with vertical wells was marginally successful in developing the fractured Austin Chalk reservoir. The development of horizontal techniques greatly enhanced the productivity of the wells by intersecting multiple fracture sets. As a result, there has been an unprecedented period of sustained development in this reservoir evidenced by the ongoing activity that is now in its twelfth year and still going strongly.

The Austin Chalk reservoir is a tight, fine-grained, fossiliferous, fractured carbonate interbedded with shales and marls that was deposited in a quiet open marine environment during a period of marine transgression. The main productive chalk trend lies at a depth ranging from 1675 to 4880 meters. The thickness of the Austin Chalk ranges from 15 to more than 180 meters with specific targets within this interval. Even though the overall characteristics of the chalk are similar across the trend, variability of the reservoir within these targets exists and results in a wide range of production profiles.

The purpose of this paper will be to highlight some of these variations by presenting four case studies covering different reservoir characteristics that are currently being exploited with horizontal wells by Union Pacific Resources (UPR). These areas are characterized as follows: 1) Up-dip, shallow oil play (Giddings Field), 2) Medium depths, oil/condensate/gas play (Brookeland Field), 3) Down dip, deep oil-gas play (Master’s Creek Field), and 4) Down-dip, deep gas play (Giddings Field).

The horizontal wellbore is drilled either as a single lateral, as stacked laterals drilled in the same direction yet testing different zones, as opposing laterals drilled up and down dip from the same surface location, or any combination of the above. The azimuth of the lateral is perpendicular to the expected fracture orientation for each area. Lateral lengths vary from 760 to over 1830 meters. Targeting of the prospective zones within the Austin Chalk is aided by using a down hole measurement while drilling (mwd) gamma ray tool. Generally the wells are completed open hole, but in some areas a liner is run through the curve to the end of the lateral.

The Austin Chalk shallow play of Giddings Field is located in Milam and Robertson counties in central Texas. The average depth of the productive interval is approximately 1700 meters. Total chalk thickness ranges from 27 to 40 meters and the targeted interval is 3 to7 meters. Porosity values of 9 to 12% and 3 to 10 Ohms resistivity characterize this targeted interval. Additional laterals are drilled in the Buda and Georgetown formations that are located stratigraphically below the Austin Chalk and separated by a thick, deep marine shale. All UPR wells in this area are drilled horizontally and perpendicular to the N70°E fracture orientation. These wells typically consist of six stacked, opposing laterals with lengths of up to 1800 meters each.

At Brookeland Field in Jasper and Newton counties of SE Texas, the Austin Chalk is 150 to 200 meters thick with the “A” zone as the primary target. This zone has a total thickness of 10 to 30 meters with a targeted interval of 9 meters. The productive interval lies at a depth between 3000 and 4000 meters. A typical well will have opposing laterals drilled in the target zone perpendicular to the N72°E fracture orientation, with each lateral length being 1500 meters. Porosity values of 2% and resistivity values of 25 Ohms are typical of the Austin Chalk reservoir in this field.

The “A” zone of the Austin Chalk is also the primary target in Master’s Creek Field, Vernon and Rapides parishes, Louisiana. This interval lies at a depth of 4115 to 4850 meters and has an overall thickness of 137 meters. The thickness of the “A” zone ranges from 37 to 43 meters with a 12 meter thick target. Porosity values range from 6 to 12% and resistivity values range from 5 to 20 Ohms. The wells are drilled perpendicular to the due east-west fracture orientation with opposing laterals each 910 to 1220 meters in length. Production is influenced by small scale faulting and fracturing, attributed to differential compaction of the underlying Tuscaloosa clastic wedge, down-dip of the Lower Cretaceous shelf margin.

The final area is the down-dip, deep gas play of Giddings Field in Washington and Austin counties of central Texas. Here, the Austin Chalk lies at a depth of 3810 to 4900 meters with a total thickness from 90 to 125 meters. The primary target, the “B” zone, is 7 to 15 meters thick and is characterized by less than 3% porosity and resistivity values greater than 40 Ohms. The wells are drilled perpendicular to the N65°E fracture strike as either a single lateral 1525 meters in length, or as opposing laterals, each 915 to 1220 meters long. Production is generally dry gas with small amounts of water and completions are based on the quantity and quality of shows encountered while drilling.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado