PEROC, Formation

TERTIARY (Oligocene and upper Eocene)

State of Zulia, Venezuela

Authors of name: Geologists of the Standard Oil Company of Venezuela (Creole Petroleum Corporation) and the Richmond Exploration Company.

Original reference: E. Mencher et al. (Correlation chart), 1951.

Original description: none published.

The name "Peroc" comes from the Peroc-1 well, which is located near the northern limit of the District of Perijá, about 14 kilometers northeast of La Villa del Rosario.

The first use of the term Peroc, as a formational name (not published), may be credited to Creole geologists, and dates back to 1930 when the Peroc well was drilled. In its initial use, the term was synonymous with the name El Fausto; however, the term came into disuse when the name El Fausto became widely recognized and accepted.

A chain of events since the 1930's has caused the name to be placed in use by Richmond geologists, with somewhat modified meaning, restricting it to the basal formational unit of the El Fausto group. The unit currently called the Peroc, was defined in 1946, its definition coinciding with change of the El Fausto sequence from formation to group status (not published). At first, this new formation was known by Richmond geologists as the "Perijá formation". However, use of the name "Perijá " in published literature, referring to pre-Cambrian or Paleozoic metamorphosed rocks in the Serranía de Perijá, claims priority. The name Peroc was consequently substituted, and is the name that is found in the correlation chart of the National Petroleum Convention, 1951. Thus, after a decade of disuse, the term Peroc was reinstated by mutual consent between Creole and Richmond with meaning limited by new definition to a considerably smaller sedimentary sequence.

The Peroc type section defined by Richmond geologists, as a result of the subdivision of the El Fausto, initially was established in the Peroc-1 well. It appears from later study that part of the overlying Macoa formation, not typically developed at Peroc-1, may be erroneously included in the 3450-5250 foot interval outlined for the Peroc unit. In order to remain consistent with the intended and pre-established usage, the type section of the Peroc is transferred from the Peroc-1 well, to the well Zulia 20D-1 (Z20-1), located about 20 kilometers farther south, where both the upper and lower boundaries of the Peroc unit are distinctive. The Peroc formation is represented by the interval 2400-4535 in the Zulia 20D-1 well. About 50% of the interval was cored.

The Peroc formation consists largely of mottled or varicolored claystones and siltstones, the colors commonly ranging through olive-gray, light olive gray, brownish-gray, pale brown, dark yellowish brown, and, in places, hues of reds and purples. The latter colors characteristically include pale red, grayish red purple, dusky-red, and very dark red. Claystone beds without evidence of mottling, ranging in color from light olive gray to dusky yellowish green are also recorded. Sandstones are rare, thin, and irregularly distributed through the section. Sporadic occurrence of anhydrite, the common presence of lignitic material, and numerous siderite pellets, commonly of yellovish and olive-brown to black color, are typical of the Peroc formation. Pyrite occurs in appreciable abundance. Heavy fractions include ilmenite and zircon, and lesser amounts of rutile, titanite, and tourmaline. Garnet and epitote are rare. The thickness of the Peroc formation recorded at the Zulia 20D-1 type section is about 2120 feet (646 m.) after correcting for dip. Such a thickness is nearly average.

In most of the mountain front area, the Peroc formation rests with conformity on sandstones of the La Sierra formation. The formation boundary is generally transitional, from sandstones and argillaceous sandstones of the La Sierra into sandy claystones in the Basal Peroc. As observed from well samples and electric logs, the gradation generally is quite abrupt, not involving more than a few feet of section. Scarcity of Peroc formation exposures has prevented examination of the Peroc-La Sierra boundary in surface outcrop.

In the vicinity of the Totumo arch, the Peroc claystones unconformably lap upon the eroded edges of Cretaceous formations; they transgress far enough upon the arch to rest, in places, upon crystalline rocks of pre-Cretaceous age. A thin basal sandstone may be present, but the La Sierra sandstones appear to be missing in part of this area. It is not clear if the absence of the La Sierra is caused by erosion or by non-deposition. Possible erosion of the La Sierra imposes a condition that, locally around periphery of the arch, implies unconformable conditions between the Peroc and the La Sierra.

At its upper limit, the Peroc formation grades into the greenish, somewhat fossiliferous claystones of the Macoa formation. In the Zulia 20D-1 Well, where the Macoa-Peroc contact interval was cored, it is possible to pick the formation boundary with considerable accuracy.

The Peroc formation is recognized in subsurface sections throughout an area extending from Peroc-1 eastward into the wells Zulia 14D-1 and 15G-1, and southward about 40 kilometers into the well Zulia 36E-1. It would be feasible seemingly, to extend use of the name into the Boscán oilfield area as well, where the name Peroc might appropriately designate the claystones currently referred to the Icotea formation. The lithology of the Icotea claystones at Boscán is intermediate between claystones of the "type" Icotea of the Lake Maracalbo area and those of the Peroc, so that the most appropriate nomenclature at Boscán may be a question of choice. From a sectional standpoint, the Peroc is conceived to occupy a similar position to the Icotea claystones at Boscán, in a generally regressive sequence. By implication, the Icotea at Boscán should be partly correlative but generally a little younger than the Peroc.

The similarity of lihology and sectional position also suggests approximate correlation between the Peroc formation and the Orumo formation of the vicinity of the Ríos Guasare and Cachirí in northern Zulia. Correlative strata are probably found within the León shale or upper Carbonera formation sediments of the Río de Oro area, where the lithologic and sectional affinity seems to be with the León.

The age of the formation has commonly been called upper Oligocene because of assumed time correlation with the Icotea formation. Position of the Peroc in the regressive stratigraphic sequence, however, implies that some of the lowermost beds in the Peroc represent continental (probably lagoonal) sedimentation nearly contemporaneous with maximum transgression of the marine Las Flores interval, and therefore are likely to be upper Eocene. Overlying beds in the Peroc sequence represent a similar lagoonal environment, of Oligocene age. Age limit at the top of the unit is imposed by the superjacent La Rosa-Macoa cycle which is generally accepted to be lower Miocene.

Fossils are restricted to rare occurrence of Miliammina fusca (Brady) ?, a siliceous foraminifer, and sporadic fish remains and shell debris. No fossils considered to be diagnostic of age have been encountered.

John B. Miller