The Lowdown on Low-Resistivity Pay Austin Boyd Evaluating low-resistivity pay requires interpreters to discard the notion Harold Darling that water saturations above 50% are not economic. Various tools and Jacques Tabanou Sugar Land, Texas, USA techniques have been developed to assess these frequently bypassed zones, but there are no shortcuts to arriving at the correct petrophysi- Bob Davis Bruce Lyon cal answer. New Orleans, Louisiana, USA Lamination of beds Shale clasts Clay-lined burrows ■ Charles Flaum Clays are the pri- mary cause of low- Ridgefield, Connecticut, USA resistivity pay and can form during James Klein and after deposi- ARCO Exploration and tion. They are dis- tributed in the for- Production Technology 0.5 in mation as laminar Plano, Texas shales, dispersed clays and struc- Pore fillings Pore linings Clay grains Robert M. Sneider tural clays. Other Robert M. Sneider Exploration, Inc. causes of low-resis- tivity pay include Houston, Texas small grain size, as in intervals of Alan Sibbit igneous and meta- Kuala Lumpur, Malaysia morphic rock frag- ments, and con- ductive minerals Julian Singer Burrowed sand Ash shards Conductive pyrite like pyrite. New Delhi, India For help in preparation of this article, thanks to Jay Tittman, consultant, Danbury, Connecticut, USA; Bar- bara Anderson, Ian Bryant, Darwin Ellis, Mike Herron, Bob Kleinberg, Raghu Ramamoorthy, Pabitra Sen, 0.25 mm Chris Straley, Schlumberger-Doll Research, Ridgefield, Connecticut; David Allen, Kees Castelijns, Andrew Kirk- wood and Andre Orban, Schlumberger Wireline & Test- When Conrad and Marcel Schlumberger low oil prices driving the reexploration of ing, Sugar Land, Texas, USA; Steve Bonner and Trevor Burgess, Anadrill, Sugar Land, Texas; Dale Logan, invented the technique of well logging, low- mature fields, methods of interpreting low- Schlumberger Wireline & Testing, Roswell, New Mexico, resistivity pay was, practically speaking, a resistivity pay have proliferated. USA; and Pierre Berger, GeoQuest, Bangkok, Thailand. contradiction in terms. Their pioneering This article examines the causes of low- research hinged on the principle that gas- or resistivity pay in sands, then explores the oil-filled rocks have a higher resistivity than tools and techniques that have been devel- water-filled rocks. Through the years, how- oped to evaluate such zones. A case study ever, low-resistivity pay has become recog- shows how log/core integration helps pin- nized as a worldwide phenomenon, occur- point the causes of low-resistivity pay in the ring in basins from the North Sea and Gandhar field in India. Indonesia to West Africa and Alaska. With Generally, deep-resistivity logs in low- resistivity pay read 0.5 to 5 ohm-m. “Low 4 Oilfield Review Lowstand basin floor fan complex A Leveed channel Overbank complex deposits Overbank ■The most com- The cation exchange capacity, or CEC, mon depositional expressed in units of milliequivalent3 per environments for low-resistivity pay: 100 grams of dry clay, measures the ability A) Lowstand basin of a clay to release cations. Clays with a floor fan complexes high CEC will have a greater impact on low- B) Deep water ering resistivity than those with a low CEC. levee-channel For example, montmorillonite, also known complexes and B overbank deposits as smectite, has a CEC of 80 to 150 Transgressive C) Transgressive- meq/100 g whereas the CEC of kaolinite is marine sands marine sands only 3 to 15 meq/100 g. D) Lower parts Clays are distributed in the formation (toes) of delta front deposits and lami- three ways: nated silt-shale- • laminar shales—shale layers between sand intervals in sand layers the upper parts of • dispersed clays—clays throughout the alluvial and dis- sand, coating the sand grains or filling the tributary channels. (Adapted from Dar- pore space between sand grains ling HL and Sneider • structural clays—clay grains or nodules in RM: “Productive Low the formation matrix. C Resistivity Well Logs Laminar shales form during deposition, Alluvial Distributary Delta front “toes” and of the Offshore Gulf interspersed in otherwise clean sands (left). channel channel shingled turbidites of Mexico: Causes and Analysis,” in In the Gulf Coast, USA, finely layered sand- reference 1.) stone-shale intervals, or thin beds, make up In this article, AIT (Array Induction Imager Tool), ARC5 (Array Resistivity Compensated), CBT (Cement Bond Tool), CDR (Compensated Dual Resistivity tool), CMR (Combinable Magnetic Resonance tool), CNL (Compen- sated Neutron Log), DLL (Dual Laterolog Resistivity), ELAN (Elemental Log Analysis), EPT (Electromagnetic Propagation Tool), FMI (Fullbore Formation MicroIm- ager), Formation MicroScanner, GeoFrame, GLT (Geo- chemical Logging Tool), Litho-Density, IPL (Integrated Porosity Lithology), MicroSFL, NGS (Natural Gamma Ray D Spectrometry tool), Phasor, RAB (Resistivity-at-the-Bit tool), SFL (Spherically Focused Resistivity), SHARP (Synergetic High-Resolution Analysis and Reconstruction contrast” is often used in conjunction with clay contributes to low-resistivity readings for Petrophysical Parameters) and TDT (Thermal Decay low resistivity, indicating a lack of resistivity depends on the type, volume and distribu- Time) are marks of Schlumberger. Sun is a mark of Sun Microsystems, Inc. contrast between sands and adjacent shales. tion of clay in the formation. 1. Moore D (ed): Productive Low Resistivity Well Logs Although not the focus of this article, low- Clay minerals have a substantial negative of the Offshore Gulf of Mexico. New Orleans, contrast pay occurs mainly when formation surface charge that causes log resistivity val- Louisiana, USA: Houston and New Orleans Geologi- waters are fresh or of low salinity. As a ues to plummet.2 This negative surface cal Societies, 1993. 2. Scala C: “Archie III: Electrical Conduction in Shaly result, resistivity values are not necessarily charge—the result of substitution in the clay Sands,” Oilfield Review 1, no. 3 (October 1989): low, but there is little resistivity contrast lattice of atoms with lower positive valence 43-53. between oil and water zones. —attracts cations such as Na+ and K+ when 3. One milliequivalent equals 6 x 1020 atoms. Because of its inherent conductivity, clay, the clay is dry. When the clay is immersed and hence shale, is the primary cause of in water, cations are released, increasing the low-resistivity pay (previous page).1 How water conductivity. Autumn 1995 5 Evaluated Gas Pay Potential Gas Pay Spherically Focused Density Porosity Short Normal Resistivity MDEN=2.68 Resistivity Spontaneous Potential 60 0 0.2 ohm-m 20 -160 40 0.2 ohm-m 20 Compensated Depth, m Spontaneous Potential 6FF40 Induction Total Gamma Ray Deep Induction Neutron Porosity -160 40 0.2 20 0 GAPI 150 0.2 ohm-m 20 60 p.u. 0 X100 X200 ■Left: Induction Electrical Survey logs run in 1960 in a thinly bedded, gas-bearing section of the Vicksburg formation in south Texas, USA. Net pay is 7 ft. Right: Conventional triple combo—neutron, density and gamma ray tools—run in 1993 in a well offset 100 ft from the original 1960 well. Net pay is 14 ft. about half the low-resistivity zones.4 Many can be produced (see “Low-Resistivity Pay fragments—all fine grained— mimic the log logging tools lack the vertical resolution to in the Gandhar Field,” page 8). signature of clays, featuring high gamma ray, resolve resistivity values for individual thin Structural clays occur when framework low resistivity and little or no spontaneous beds of sand and shale. Instead, the tools grains and fragments of shale or claystone, potential (SP). Unlike thin beds, this type of give an average resistivity measurement over with a grain size equal to or larger than the low-resistivity pay can vary in thickness the bedded sequence, lower in some zones, framework grains, are deposited simultane- from millimeters to hundreds of meters. higher in others. ously. Alternatively, in the case of selective Finally, sands with more than 7% by vol- Intervals with dispersed clays are formed replacement, diagenesis can transform ume of pyrite, which has a conductivity during the deposition of individual clay par- framework grains, like feldspar, into clay. greater than or equal to that of formation ticles or masses of clay. Dispersed clays can Unlike dispersed clays, structural clays act water, also produce low-resistivity readings.5 result from postdepositional processes, such as framework grains without altering reser- This type of low-resistivity pay is considered as burrowing and diagenesis. The size differ- voir properties. None of the pore space is rare. ence between dispersed clay grains and occupied by clay. The challenge for interpreting low-resistiv- framework grains allows the dispersed clay Other causes of low-resistivity pay include ity sands hinges on extracting the correct grains to line or fill the pore throats between small grain size and conductive minerals measurement of formation resistivity, esti- framework grains. When clay coats the sand like pyrite. Small grain size can result in low mating shaliness and then accurately deriv- grains, the irreducible water saturation of resistivity values over an interval, despite ing water saturation, typically obtained from the formation increases, dramatically lower- uniform mineralogy and clay content. The some modification of Archie’s law.6 ing resistivity values. If such zones are com- increased surface area associated with finer Improved vertical resolution of logging pleted, however, water-free hydrocarbons grains holds more irreducible water, and, as tools and data processing techniques are with clay-coated grains, the increasing helping to tackle thin beds. Nuclear mag- water saturation reduces resistivity readings. 6 Intervals of igneous and metamorphic rock Oilfield Review Potential Gas Pay 2 ft [0.6 m] and 4 ft [1.2 m]. The FMI tool images the borehole with an array of 192 Spontaneous Potential button sensors mounted on four pads and four flaps.8 It has a vertical resolution of -160 MV 40 0.2 in. [5 mm]. Successive improvements in resolving thin APS Capture Density Porosity Cross-Section (SIGF) MDEN=2.68 (DPO) beds are strikingly visible in a series of logs 10 c.u.