Geol 404 1st Edition Lecture 21 Outline of Last Lecture I. Clastic ReservoirsII. 4th Order HeterogeneityIII. Depositional TextureIV. CompactionV. Pressure SolutionVI. Porosity TypesOutline of Current Lecture VII. Alteration and DissolutionVIII. Porosity DevelopmentIX. Burial vs PorosityX. PermeabilityXI. Core Plug OrientationXII. 3rd Order Heterogeneity XIII. Laminae and LaminaXIV.Clastic ReservoirsCurrent LectureVIII. Alteration and Dissolutiona. Certain minerals are very susceptible to chemical break-down or dissolution in surface conditions of high temperature and pressure b. Feldspar grains and calcite cements are both relatively unstable under such conditions, and given enough depth and time, their grains dissolve and create “secondary” porosity and permeabilityIX. Porosity Development a. Is the “Storage Capacity” of a rockb. Porosity is the ratio of void (or pore volume) to macroscopic or bulk volume.c. For most naturally occurring sandstones, porosity is between 0.10 and 0.40% although on rare occasions, values outside this range have been observed. d. The porosity’s of petroleum reservoirs range from 5 to 40% but most frequently are between 10 and 20%X. Burial vs PorosityThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.a. In general, porosities tend to be lower in deeper and older rocks. b. This decrease is primarily due to i. – Overburden – pressure solution, – Diagenesis and cementation c. Shales closely follow the same porosity depth trend as sandstones. d. For example, a Recent mud may have a porosity of 40%. e. It decreases rapidly with depth and overburden pressure until, at a depth of 10,000’ normal porosities would be less than 7%. f. Shales are plastic and therefore, compress more easily than sands.XI. Permeabilitya. The second essential requirement for a reservoir rock is permeability and the best way to visualize it is as “flow capacity”. b. The original work on permeability was carried out by H. Darcy (1856). c. Fittingly the unit of permeability is the “Darcy”. d. A Darcy is defined as the permeability that allows a fluid of 1 centipoise (cp) viscosity to flow at a velocity of 1 cm/s for a pressure drop of 1 atm/cm. e. Because most reservoirs have permeabilities less than a Darcy, the millidarcy (md) is commonly used.f. Measurement of permeability by commercial companies such as Corelab. is by means of a permeameter. g. Gas is forced through a prepared rock sample which has been cut as a plug from a wholecore. h. Formula’s for permeability and flow Rate are given opposite.XII. Core Plug Orientationa. For optimum results, core plugs should be cut either parallel or perpendicular to the bedding planes or lamination b. Plugs that intercept bedding planes or any other form of discontinuity will not give a truemeasure of these parametersXIII. 3rd Order Heterogeneitya. Mesoscopic Scalei. This scale encompasses cores and well logs. ii. Such heterogeneities include bedding and lithologic types, stratification styles, and nature of bedding contactsXIV. Laminae and Laminaa. From smallest to largest, the component layers of a clastic sand waves, dune, ripples etc.are:i. – laminae, – lamina-sets, – beds – bed-sets.b. Different arrangements of these layers characterize different types of sedimentary bodies and identify different depositional processes c. The four kinds of layer are genetically similar, when compared with each other. d. Where they differ is in areal extent and interval of time for formatione. Grain-size within each laminae varies considerably depending on the way the laminae was deposited and the sorting process involved. f. Avalanche facies on fore-sets tend to be downward coarsening but beds with more oscillatory flow can have a coarsening-up texture depending on how much suspended mud is in the system. g. To measure these variations and to get their respective permeability’s, core plugs in three axes may be takenXV. Clastic Reservoirsa. Grain size and shaliness have long been used by geologists to help derive sedimentological models and correlations. b. The illustrations show how the gamma-ray responds to the variations in grain size. c. Many geologists when describing a gamma-ray (or SP) curve will refer to a “bell-shaped” or “X-mass tree” shape inferring a fining-up sequence. d. Conversely, a “funnel” or “inverted X-mass tree” shape signifies a coarsening-up sequence.e. All the data from our 4th and 3rd order heterogeneity studies how a a geologist and reservoir engineer can get a realistic idea how a reservoir might (or might not!)