ABSTRACT

Unconformities have been significant in the formation of Woodbine and Tuscaloosa traps in the up dip areas of east Texas and north Louisiana. Prominent among them are East Texas Field and Delhi-Big Creek Field in north Louisiana. Unconformities have also been important in the down dip areas of East Texax in traps at Sugar Creek, Seven Oaks-Hortense-Leggett, and more recently, Double A Wells. Their importance has been underestimated in west-central Louisiana. Unconformities are important because they record significant erosion of previous deposits and their re-deposition in down-dip areas. Additionally, unconformities help form traps.

West central Louisiana has often been referred to as a "no sand" area as far as the Woodbine-Tuscaloosa is concerned. One of the reasons for this conclusion is a failure to recognize that the thick shale over large parts of west-central Louisiana up dip of the Lower Cretaceous reef is the Rapides Shale, a younger-aged unit that unconformably overlies the Woodbine-Eagleford and Tuscaloosa sequences (WETS).

Both Forgotson (1958) and Halbouty (1991) recognized that the previously deposited sand-bearing WETS have been deposited across the Sabine Uplift and subsequently have been stripped off over approximately 8,300 square miles in west-central Louisiana. It may be more accurate to say today the shelf area in west central Louisiana is a "no sand area," but it was not always so.

So what does this suggest for more successful Woodbine-Tuscaloosa exploration in west-central Louisiana?

Part of the answer is in finding out the exact age of the erosion on the shelf. To do that you have to go across the shelf edge and look into the basin.

Seaward of the Lower Cretaceous reef, where sufficient accommodation space existed, a remnant of the older WETS can be seen, as well as a sequence not seen in most places on the shelf. It is important to distinguish between the two sequences. The first, older sequence is a distal facies of the WETS that was deposited and subsequently eroded over most of the west-central Louisiana shelf. The second sequence is a low-stand sequence that records deposition of eroded WETS sediments off the old shelf edge. The age of this unconformity is post-WETS and pre-Rapides Shale-Austin Chalk.

Drilling immediately in front of the Lower Cretaceous reef at this location will likely encounter only the older WETS. Prospective reworked sands deposited by deltas and other shallow marine processes are associated with the younger post-WETS low-stand deposits.

The "no sand" Rapides shale dominates the up dip section on the shelf. An exception to this is a series of sands that are located below the Base Rapides unconformity and above the Top Lower Cretaceous unconformity. These sands look like, but occupy a different, younger stratigraphic position than the well known Basal Tuscaloosa sands of northeast Louisiana and Mississippi. These sands may represent "incised valley fills" which are related to more extensive post-WETS, pre Rapides Shale-Austin Chalk low-stand deposits farther down-dip over the shelf edge.

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The sand-bearing section found to the east in Avoyelles and St. Landry Parishes can be correlated into Rapides and even Vernon Parish. In some cases wells with "no sand" were drilled either too far up-dip or down-dip to hit sand. Wells drilled too far updip missed the sand-bearing wedge entirely. Wells drilled down-dip of the post-WETS and Tuscaloosa wedge were too distal for good quality sand.

Wells with little to no sand may be near misses rather than data points that condemn townships. With well density of one per township, the area down-dip of the reef is hardly evaluated.

There is obviously more to the successful exploration model. Other elements of the exploration model include structural noses for sufficient column height, depositional environments and diagenetic effects on the reservoir extent and quality.

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