Geology and Coproduction Potential of Submarine-Fan Deposits Along the Gulf Coast of East Texas and Louisiana
Authors: Jackson Mary L.W. , Light Malcolm P.R. , Ayers Walter B. 
Published: December 2007

Summary
Four reservoirs containing dispersed gas were examined for their coproduction potential. Reservoirs in Port Acres field (Texas) and Ellis field (Louisiana) produce from the Hackberry member of the Oligocene Frio formation, and two reservoirs in Esther field (Louisiana) produced from the lower Miocene Planulina zone. Log-pattern and lithofacies maps, together with stratigraphic position, suggest that the reservoirs are in ancient submarine-fan deposits. Dip-elongate, channel-fill sands are characteristic; reservoir sands pinch out along the strike. Growth faults, common in the submarine slope setting, form updip and downdip boundaries, producing combination traps.
In Ellis field, coproduction accounts for 300 Mcf/D [8.5 × 103 m3/d] of gas. Port Acres field contains the largest remaining reserves, but other technical and economic factors limit coproduction potential there. Recent drilling has extended primary production and delayed coproduction in the Esther field.

Introduction
During conventional production from a water-drive gas reservoir, mobile gas is removed from the gas cap. A considerable amount of dispersed gas remains in the water, however, after the field has watered. out.1 This dispersed gas can be recovered through coproduction (the simultaneous production of gas and water). During coproduction, large amounts of water are pumped to the surface, lowering reservoir pressure and causing dispersed gas to expand; the gas then moves to the coproducing well and is produced with the water.2 If the reservoir has sufficient residual pressure and the pumped water can be disposed of easily, extended production of a field may be economically viable.1 To avoid costly well re-entry or drilling of new wells, coproduction is best initiated in a watered-out reservoir before the wells have been plugged.
Four reservoirs with coproduction potential were selected for detailed study. The resrvoirs are in Port Acres, Ellis, and Esther fields (Fig. 1). The lower Hackberry and Nodosaria 3 reservoirs, in Port Acres and Ellis fields, respectively, are productive from the Hackberry member of the Frio formation, and the two Planulina zone reservoirs in Esther field are productive from lower Miocene sands. Several factors that determine the suitability of a reservoir for coproduction - such as porosity, permeability, and reservoir volume3 - relate to the geology of the reservoir. Therefore, for each reservoir selected for study, all available data were used to interpret the depositional setting of the reservoir sand, the trapping mechanisms, and the volume of the reservoir. Original gas in place and remaining reserves were calculated, and the coproduction potential of each field was assessed.

Hackberry Trend
The Frio formation (Oligocene) is a major progradational wedge of predominantly sandy sediment that extends from Texas to Louisiana. Previous workers divided the Frio into three units by log character. In this interpretation, the Hackberry member is laterally equivalent to middle Frio sediments.4 The upper boundary of the Hackberry sequence is marked by Marginulina texana, and for the purpose of this paper, the lower Hackberry will include the Nodosaria blanpiedi zone, although there is some dispute over the lower boundary of the Hackberry5 (Fig. 2).
The Hackberry member was probably deposited as a submarine-fan complex6; it is composed mainly of turbidite, or gravity-flow, deposits. The Hackberry embayment, containing Hackberry submarine fans, extends from southeast Texas eastward to south-central Louisiana4 (Fig. 1). The updip limit of Hackberry sediments is the Hartburg flexure, which is more or less coincident with the oldest growth faults in the Frio.5 The unit thickens basinward to more than 3,000 ft [900 m].7 The lower Hackberry is relatively sand-rich, with nearly continuous sand sequences up to 800 ft [240 m] thick; the upper Hackberry is almost entirely mud.
Depositional channel-sand axes of the Hackberry trend northwest/southeast in Texas and north/south in Louisiana.4 Hackberry reservoir sands are highly lenticular and dip-elongate; they are an important producing trend in the gulf coast.5
A model of submarine-fan subenvironments8 linked with spontaneous-potential (SP) log patterns representing the different subenvironments4 (Fig. 3) was used to interpret the environments present in the Port Acres and Ellis Hackberry reservoirs.