Abstract

Core description, karst fabric identification and conodont biostratigraphy in the Lower Ordovician Ellenburger Group serve to significantly improve subsurface correlations, construction of geological models, and evaluation of reservoir character and potential performance. Robust modeling of geological processes and evaluation of reservoir heterogeneity are augmented by consistent utilization of this multi-phased and integrative strategy.

Cores of the Ellenburger formation (230 ft) from the Conoco Shannon #A-28 well, in Elkhorn field, Crockett County, Texas display a mixture of limestone and dolomite, as well as karst fabric (Kerans, 1988, 1989, and 1990): collapse cave roof, lower collapsed zone, cave fill, and unaltered cave floor. Karst fabrics linked to reservoir heterogeneity, are corroborated by observed fluid and pressure anomalies. Moldic to vuggy porosity and matrix intercrystalline porosity define a bimodal pore system that is correlative to ultimate “break through” of water and resultant lower oil cuts.

Stylolitization, stylonodular fabric, thin–bedded, nonporous and impermeable dolomite and ductile greenish gray shales, silicified and ferruginous zones, and silicified terra rosa limit vertical permeability. Crackle breccia or mosaic fractures, short subvertical fractures, and matrix–supported collapse breccias introduce additional heterogeneity.

The absence of the upper Deltatus–Dianae zone through Ganada–Anidus and Aranda–Jaanussoni zones indicates post–Ellenburger erosion. Elements in the Deltatus–Dianae Zone occur in the uppermost core. The subjacent Quadraplicatus–Rex zonal indicators are recovered above elements from the Manitouensis–Bransoni Zone. No elements restricted to zones older than the Manitouensis–Bransoni Zone are present.

Documentation of karst fabric and the conodont sequence are a solid basis to calibrate older insoluble residue data within a field and interpret the variable wireline response in the Ellenburger. Stratigraphic “leaking” through karst-formed vertical conduits has juxtaposed reworked older or younger late Early and Middle Ordovician transgressive sediments with in situ conodonts. The “leaked” faunas have zonal differences indicating several different karsting episodes. Karst features in the Shannon #A-28 well were effectively sealed by medial Middle Ordovician time, since no conodonts younger than Fauna 4 of the overlying Whiterockian Middle Ordovician are present.

The Quadraplicatus–Rex Zone is very thin compared to reference sections in Oklahoma, the Llano Uplift, and the Franklin Mountains, Texas. Its base immediately overlies the terra rosa interval, while the top of the subjacent Manitouensis–Bransoni zone lies immediately below the base of the terra rosa. Further study may reveal the potential of regional superimposed karsting events.