ABSTRACT

Approximately a million barrels of oil and two billion cubic feet (7.7X10⁷m³) of gas have been produced from the Vivian Field since its discovery in June of 1981. Ultimate recoverable reserves after water flooding are estimated to be 5.1 million barrels of oil and 8.3 billion cubic feet (2.4X10⁸m³) of gas. The field is a stratigraphic trap resulting from porous, permeable grainstones pinching-out into muddy limestones and shale. It is located on the northwestern flank of the Caddo-Pine Island Structure in Caddo Parish, Louisiana (Figure 1) and has 56 productive wells, 52 of which produce from one or more reservoirs in the Pettet "B" interval (Figure 2). (The Pettet "B" is a term used by the operator of the field to denote the second of four limestone units encountered when drilling through the Lower Cretaceous Sligo Formation in the vicinity of Vivian Field.) The reservoir rock is primarily skeletal-rich grainstone, rarely oolitic grainstone. The occurrence of these high-energy deposits at this location indicates that the broad Sligo platform (Herrmann, 1971) was divided by an oolite-shoal complex into a platform lagoon on the landward side of the grainstones and an outer platform on the basinward side (Figure 1, inset). A similar situation was described by Bebout et al (1981) for the Sligo of South Texas.

The Pettet "B" interval contains eight microfacies: oolitic grainstone, skeletal grainstone, skeletal-oolitic grainstone, skeletal packstone, oolitic packstone, skeletal-oolitic packstone, skeletal wackestone and shale. Three depositional environments are represented by these microfacies. A high-energy environment is represented by well-sorted oolitic grainstone of which there are two subfacies: coarse-grained oolite and medium-grained oolite. In both varieties ooid nuclei are skeletal fragments. Only the largest grains are uncoated, indicating that local wave or tide energy was great enough to keep all but the large grains in motion. The oolitic grainstone microfacies likely was deposited in a very shallow subtidal to intertidal environment.

The skeletal grainstone represents a moderate-energy belt along the outer-platform flank of the oolite shoal. Allochems in this microfacies are primarily uncoated molluscan shell fragments with an appreciable amount of echinoderm detritus in many samples. This microfacies was deposited in the subtidal zone where wave or current energy was great enough to winnow out mud and silt but too weak to agitate the skeletal grains.

Gradational between the skeletal and oolitic grainstones is a skeletal-oolitic grainstone microfacies. This microfacies accumulated where wave or current energy was great enough to agitate the smaller skeletal grains but not the larger ones, producing some autochthonous ooids and leaving other grains uncoated (Flugel, 1982).

A low-energy environment is represented by highly burrowed skeletal wackestone and silty shale microfacies. Allochems in the skeletal wackestone are molluscan and echinoderm detritus and benthonic foraminiferas. Quartz silt can be abundant in the shale. These microfacies are commonly finely intercalated and tend to grade into one another. They indicate quiet-water sedimentation on the outer platform.

The transition from the higher-energy grainstones to the lower-energy wackestone and shale is represented by the three varieties of packstone. Packstone comprises a very small percentage of the overall volume of rock in the Pettet "B", indicating that the transition was abrupt. By far the most abundant packstone microfacies is skeletal packstone; the next most abundant is skeletal-oolitic packstone; and rarely there is oolitic packstone.

The common vertical sequence from bottom to top in the Pettet "B" is oolitic grainstone--skeletal grainstone--packstone--wackstone--shale--wackestone--packstone--skeletal grainstone--oolitic grainstone. This sequence represents a cycle of sedimentation. The lower grainstone unit is called the B2 and the upper grainstone unit the B1. Both grainstone units have two separate zones of permeability so that there are an Upper B1, Lower B1, Upper B2 and Lower B2 reservoirs (Figure 3).

Reservoir-grade permeability of 0.5 millidarcies or greater is restricted to the grainstone microfacies in the Pettet "B". Normally this occurs in the skeletal and skeletal-oolitic grainstone microfacies where permeabilities can be as high as 80 millidarcies. A portion of the Lower B2, however, does have lower-limit reservoir permeability in a medium-grained oolitic grainstone. Porosities associated with reservoir-grade permeabilities range from 9 to 12 percent.

Although many factors contribute to the distribution of porosity and permeability in the field, by far the most important one is the distribution of the skeletal-rich grainstones in which porosity has been enhanced and retained. Porosity enhancement is the result of early dissolution of aragonite mollusc shells due to fresh-water influx which created skeletal molds. The occurrence of echinoderm overgrowths within some skeletal molds attests to the early development of this porosity. Porosity was retained as the result of both early fresh-water-phreatic rim cements and early structural movement. Early rim cement prevented compaction, keeping interparticle and moldic porosity open. Early structural movement placed this porous grainstone in an updip position favorable for early hydrocarbon accumulation, which