Thin-Bedded Peritidal Reservoirs Of The Silurian Upper Interlake Group, Nesson Anticline Area, Williston Basin, North Dakota

ABSTRACT
Formation evaluation of the Upper Interlake Group (Silurian) has been hampered by several factors including the thin-bedded cyclic nature of these deposits. The depositional cycles are thin (0. 5-6 m), shallowing-upwards, peritidal dolomite sequences beginning in subtidal, massive mudstones (0-25% ?,001-0. 1 md); grading to intertidal rooted and desiccated mudstones (0-24% ?, 0. 01-0. 5 md); overlain by high intertidal fenestral algal boundstones (5-24% ?, 0. 01-1000 md); and capped by one of several types of supratidal aust (variable ?, K). The algal boundstones include well interconnected vuggy pore systems with low capillary entry pressures(<20 psi, laboratory air-brine) that rapidly desaturate to an irreducible Sw determined by the proportion of microsucrosic, intercrystalline porosity between the vugs. The microintercrystalline porosity, which is pervasive throughout all of the dolomite mudstone fades as well, does not desaturate within the range of buoyancy pressures (<100 psi) expected for any structures in this basin. Log calculations almost universally condemn the entire Interlake section as >60% apparent Sw. Because the productive lithologies are typically much less than 1 meter thick and their porosity range and mineralogy are similar to that of the non-productive mudstones they cannot be distinguished from the surrounding mudstones using logs alone. Typical evaluation strategies based on testing zones above some minimum porosity cutoff run the serious risk of missing productive intervals that have less total porosity than the arbitrary cutoff, yet contain significant amounts of well interconnected vuggy porosity. Similarly, non-productive yet high porosity mudstones are commonly flagged as possible pay and have been included in net pay thickness calculations, leading to erroneous reserve estimates. Attempts to distinguish "secondary porosity" from logs fail because acoustic logs lack the required vertical resolution. Additionally, distinction on the basis of saturation fails because the pay zones are below the vertical resolution of deep resistivity tools, and severe shoulder bed effects result from the pervasive background of moderate to high porosity, water wet (> 85% Sw) dolomite mudstones. Currently deployed wireline technologies are limited in their ability to assess the Upper Interlake; however, certain evaluation strategies can improve pay definition. Today, high resolution processing of porosity logs can resolve most significant beds in the Interlake. Careful drill cuttings analysis and coring prospective intervals combined with geologically guided core analysis are presently the most cost effective and definitive means of evaluating upper Interlake reservoirs. With careful calibration to known lithologies, borehole imaging logs might have the resolution required to identify productive lithologies and microresistivity surveys may be successful defining the high permeability zones among them.