Naturally Fractured Reservoirs Roberto Aguilera [...]... te small compa rtment s within the reservoir that can lead to unecon omic or margin al recove ries In my opinio n a1l reservoirs cuntai n a certain alnoun t of natura l fracturing However, froIn a geolog ic and a reservo ir engine ering point of view, 1 regard as natura lly fractured reservoirs only those where the fractures have an effect, either positiv e or negativ e, on fluid tlow as sugges ted... ion proVided by Nelson (1985) is as follows: itA reservo ir fracture is a natu!a lly occurr ing Inacro scopic planar discon tinuity in rock due to deform ation or physic al diagen esis." WHA T IS A NATURALLY FRACTURED RESERVOIR? A natura lly fractur ed reservo ir is a reservoir which contai ns fractur es created by mothe r nature These natura l fractures can have a positiv e or a negativ e effect on... ed reservoir~ have been accide ntally discovere d when lookin g for some other type of reservoir Some years ago McNa ughton and Garb (1975) estima ted that ultima te recovery from produc ing fractur ed reservoirs would surpas s 40 billion stock tank barrels of oil (STllO) Today I firmly believe that this figure was very conser vative I am con vinced that there are signifi cant volum es of hydroc arbons... overed , or behind plugge d and aband oned wells or becaus e of vertica l wells that have not interce pted vertica l fractur es Figure 1-1 shows the locatio n of some impor tant natura lly fractur ed reservoirs They are found over the world, in all types of litholo gies and throug hout the geolog ic stratigraphi c colum n This is demon strated in Chapt er 5 dealin g with Case Histories all WHA T IS . a second volume covering other aspects of naturally fractured reservoirs. Chapter 1 deals with geOlogic aspects of naturally fractured reservoirs, reasons for gen- erations of fractures. Migration and Accun~ulation Sources of Information Mapping Fractured Trends Fractured Reservoirs Exploring fo r Naturally Fractured Reservoirs Subsurface Fracture Spacing Imp ~c t Fr act ures. Gas Reservoirs References CHAPTER 5 CASE HISTORIES Frac tured Sandstones· Fractured Carbona tcs Fractured Cherts . Fractured Shales Fra ctu red Basement Tight Gas Reservoirs Naturally