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Rapid Compactional Progression in Sand and Mud: Data From the Nicobar Fan, Indian Ocean

Abstract

Intergranular volume (IGV; the sum of intergranular pore space and intergranular cement volume) is a well-established concept for sand and sandstone, and constitutes a key parameter in models for porosity prediction in sandstone reservoirs. IGV of well-sorted quartzo-feldspathic sand is in the range of 40-45% at deposition and is generally believed to stabilize at around 26% by 1.5-2 km of burial. The IGV concept for muds is less refined. IGV in the range of 70-80% characterizes mud near the sediment-water interface, but the depth to compactional stabilization in muds is uncertain. Samples to evaluate the progression of compaction in the critical burial range between deposition and compactional stabilization are seldom available from oil and gas drilling. A particular obstacle to developing an IGV concept for mud concerns the challenge of isolating the effects of compaction versus cementation in porosity decline. Recent coring by IODP Expedition 362 in the Indian Ocean provides an opportunity to evaluate sand and mud compaction in the context of a texturally and compositionally homogeneous and normally pressured sediment package in the Nicobar Fan (Miocene-Pleistocene). Critical to this assessment is the unconsolidated nature of the sediments, lending some certainty to the interpretation of total porosity as representative of IGV. Shipboard analysis of porosity by a moisture and density technique reveals that compaction in both sand and mud progresses rapidly. Undisturbed and relatively mud-free sands achieve an IGV of < 40% by 50 m of burial; by 220 meters sand is fully outside the range of depositional IGV. Following this initial rapid phase, compaction slows and at 1.2 km sand still has IGV in the range of 26-38%. In muds the highest IGVs, near 80 %, are observed only in the upper 5 meters; by 60 m IGV in mud is in the range of 42-55%, higher than in associated sand. At 1.2 km, however, sand and mud have a similar range of IGV. Based on these observed trends cement volumes in excess of 40% can be confidently attributed to precipitation at burial depths less than 600 m in both sand and mud. In mud, cement volumes above 50% would indicate precipitation at burial depths less than 200 m; cement volume exceeding 60% in mud would suggest cementation at less than 30 m. The depths and mechanisms by which mudrock attains the low porosities observed in typical oil and gas shales remains an outstanding question for investigation.