Basalt sills in the 10,054-ft deep Ore-Ida well: western Snake River Plain Cenozoic basin of Idaho and Oregon

Abstract

Basalt sills intrude the late Cenozoic subsurface sediments of the western Snake River Plain in the Ontario, Oregon area. The 10,054-ft deep Ore-Ida geothermal well contains 6 basalt layers, interpreted as sills, 10-260 ft thick, totalling 840 ft, within the section from 4,500-8,050 ft depth. Cuttings are described as a ‘diabase’. Cuttings from contacts lack the reddish coloration of subaerial flows, and lack olive palagonite and black glass of sub-lacustrine flows. Geophysical logs clearly show increased resistivity of mudstone above and below the sill. Dehydration or baking of the sediment produces an aureole, typically 40-ft thick, that appears on the resistivity log as a flattened dumbbell shape centered on the highly resistive basalt. Natural gamma logs show the sharp contact of relatively low gamma count basalt compared to the much higher gamma count of mudstone. All of the sills have similar natural gamma of ~65 API. From 8,080-9,248 ft is a 1,168-ft thick basalt of uncertain structure with uniformly low gamma count (20-30 API). From 9,248-10,052 ft TD are interbedded rhyolite (220+API) and basalt (50 API). Rock chemical analysis from 9 selected depth samples of basalt cuttings (10-ft sample bags) from depths 4,620-9,690-ft are all quite similar and plot in the field of Snake River olivine tholeites (SROT) with high TiO2 (3.33 — 3.94%), and SiO2 (44.7-47%), FeO (15-15.7%) and MgO (4.7-6.4%). It is a surprise that the deep basalt cuttings at 8,390 feet have a similar chemistry to the shallower group despite dissimilar gamma counts. The high TiO2 suggests that none of these basalts are typical Columbia River Group. No cuttings could be found in the sample bags of the interbedded rhyolites indicated by the gamma logs suggesting the rhyolites may be fine ash. This study shows that some subsurface igneous rocks of the western plain formerly thought to be flows are intrusive and complicate the interpretation of cuttings, well logs, seismic data, and the origin of hydrocarbons. On resistivity logs intrusive sills show a clear expression of their surrounding aureole of altered mudstone, but careful selection of cuttings from wells and chemical analysis will be important to their understanding and correlation.

AAPG Datapages/Search and Discovery Article #90266 © 2016 AAPG Pacific Section and Rocky Mountain Section Joint Meeting, Las Vegas, Nevada, October 2-5, 2016