GAS-RESERVOIR POTENTIAL OF THE LOWER ORDOVICIAN BEEKMANTOWN GROUP, QUEBEC LOWLANDS, CANADA

The Beekmantown Group in the Quebec Lowlands was deposited as part of an extensive Early Ordovician coastal and shallow marine complex on th e eastern margin of the North American craton. The Beekmantown is stra tigraphically equivalent to the Beekmantown, Knox, Arbuckle, and Ellen burger rocks of the United States, and is subdivided into two formatio ns: the sandstone-rich Theresa Formation and the overlying dolomite-ri ch Beauharnois. Dolomites of the Beekmantown provide an important expl oration target in both the autochthon and the overlying thrust sheets of the Canadian and U.S. Appalachians. The reservoir potential of the autochthonous Beekmantown Group in the Quebec Lowlands can be determin ed from seismic data, well logs, cuttings, and petrographic analyses o f depositional and diagenetic textures. Deposition of the Beekmantown occurred along the western passive margin of the Iapetus Ocean. By the Late Ordovician, the passive margin had been transformed into a forel and basin. Faulting locally positioned Upper Ordovician Utica source r ocks against the Beekmantown and contributed to forming hydrocarbon re servoirs. The largest Beekmantown reservoir found to date is the St. F lavien field, with 7.75 bcf of original gas (methane) in place in frac tured and possibly karst-influenced allochthonous dolomites within a t hrust-fault anticline. The Beekmantown below the thrust sheets forms a northward-thinning wedge of peritidal and subtidal deposits. Seven ma jor depositional units can be distinguished in cuttings and correlated with wireline logs. Most of these units form northward-thinning sedim ent wedges and were deposited on a gently dipping ramp. Quartz sandsto nes dominate updip, whereas shallow, subtidal, pelletal to skeletal li mestones dominate downdip. A widespread blanket of shaly dolomite is t he uppermost unit of the Beekmantown, but is of poor reservoir quality . Dolomites in the Beekmantown contain vuggy, moldic, intercrystalline , and fracture porosity. Early porosity formed at the top of the major depositional units in peritidal dolomites; however, much of this poro sity was later filled by late-stage calcite cement after hydrocarbon m igration. Thus, a key to finding gas reservoirs in the autochthonous B eekmantown is to define Ordovician paleostructures in which early and continuous entrapment of hydrocarbons prevented later cementation.