Abstract: Thin-Bedded Peritidal Reservoirs of the Silurian Upper Interlake Group, Nesson Anticline Area, Williston Basin, North Dakota

Robert M. Cluff, Alan P. Byrnes, Roger W. Kolvoord, Suzanne G. Cluff, Richard F. Inden

The upper Interlake is a repetitive sequence of thin (0.5-6 m), shallowing-upwards, peritidal dolomite cycles consisting of: 1) basal massive mudstones (0-20% ^phgr, 0.001-0.1 md); 2) rooted and desiccated mudstones (0-24% ^phgr, 0.01-0.5 md); 3) fenestral algal boundstones (5-24% ^phgr, 0.01-1000 md); 4) capping supratidal crust (variable ^phgr, K). Boundstones and crusts have a well interconnected vuggy pore system with low entry pressure (P_c < 20 psi air-brine) that rapidly desaturates to low irreducible S_w (^Lt 50%). A pervasive background microsucrosic, intercrystalline porosity in all facies does not desaturate within the range of buoyancy pressures expected for any structural closures in the basin (< 100 psi; S_{w irr} > 85%).

Log calculations almost universally condemn the upper Interlake section as water wet. Because the productive lithologies have average porosity and bulk mineralogy similar to the enclosing water-wet mudstones, they cannot easily be distinguished from adjacent beds. Thus non-productive mudstone intervals are commonly flagged as "pay" and included in net pay computations, because log S_w's are not trusted and the zones meet minimum porosity cutoff criteria. Similarly, productive vuggy intervals are sometimes excluded because they fail to meet the porosity cutoff. Distinction on the basis of density-acoustic porosity separation or high resistivity usually fail because the zones are below the vertical resolution of deep induction and standard acoustic logging devices.

High resolution processing of logs, enhanced vertical resolution wireline tools, borehole imaging logs, and microresistivity devices offer the best opportunities for better formation evaluation of the upper Interlake. Selective testing, coring, and cuttings collection over short intervals also improve the quality of completion decisions.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994