James Lime Play
Geology
The lower Cretaceous James Lime horizontal play is composed of a chalk facies. Like most chalks, it is naturally fractured. Some beds are very similar to the Austin Chalk with low porosity and only fractures to serve as permeability. Other beds within the James are more like North Sea chalks with very high porosity (up to 20%) and micro-pore throats for transmissibility. This rock type has moldic porosity, where shells and forams dissolved away, as well as the chalky micro-pores. The high porosity rock can also be fractured.
Figure 9 is a good illustration to show these two end members. It should be considered that the high porosity rock is found in the Huxley and Trawick areas, whereas the lower porosity rock is found mostly in the rest of Nacogdoches County where the fracture treatments appear to work best.
The James is sandwiched between two fissile black shale formations – the Bexar Shale above, and the Pine Island Shale below. Both shale horizons are possible source rocks, as well as excellent seals. Overall, the porosity found in the James Lime pinches out up-dip, near Carthage Field (refer to the cross section, Figure 10).
Sweet-spots of porosity appear to favor geologic paleo-highs, which are often modern day geologic structural highs. Porosity tends to increase on the structural highs. For example, Trawick Field is a salt dome, which has decreasing porosity of the structure. This is also seen in our prospect areas. One technique for mapping permeability is to map positive Sp deflection off of electric logs. In many cases, good positive values are also seen on the structural highs.
One paradigm for the Huxley area has been that horizontal wells with lots of open hole section or very long laterals are better performers, but his correlation does not hold true in the Nacogdoches play. Figure 11 shows that longer lateral actually show a degredation in performance. This may be due to increased formation damage occurring as a result of the well bore being exposed to mud and cutttings for longer periods of time as the wells are drilled longer distances.
A schematic model of the James Lime reservoir is illustrated in Figure 12. The rock actually has innumerable thin healed fractures, with only occasional large open fracture sets. The large open fractures give the James wells their high Initial Potentials (IPs). Like most fractured reservoirs, these open fractures are susceptible to formation damage from drilling fluids and casing cement. The healed and partially healed fractures – where calcite crystals fill the once open space with in the fracture – tend to impart a strong anisotropic direction to the drainage area of James Lime well bores. Thus, horizontal well bores are a good strategy in the James Lime, even large open fractures are not intersected by the lateral. The model shows that the relative proportion of Austin Chalk-like rock versus North Sea Chalk-like rock (shown in orange) can dictate production performance.