SOCIETY OF PETROLEUM ENGINEERS OF AIME 6200 North Central Expressway =&i SPE 2988 Dallas, Texas THIS IS.A PREPRINT --- SUBJECT TO CORRECTION Study of the Hydrodynamic Pattern in a Sedimentary Basin Subject to Subsidence RY Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/70FM/All-70FM/SPE-2988-MS/2069932/spe-2988-ms.pdf by guest on 28 September 2021 C. Jacquin and M. J. Poulet, Institut Francais du Petrole @ Copyright 1970 American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. This paper was prepared for the 45th Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in Houstonj Tex., Oct. 4-7, 1970. Permissionto copy is restrictedto an abstract of not more than 300 words. Illustrationsmay not be copied. 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ABSTRACT tion and the order of magnitude of these Qverpre~suresagree with standard observations The structure of a sedimentarybasin is made on actual sedimentarybasins. modelized, assuming that this basin is subject to subsidenceand has the following character- 3* The circulationof water through sand istics: (1) shaped like an inverted cone with horizons occurs at very low speeds of between a circular base having a radius of 300 km, - about 1 and 10 mm/year, but the volumes of (2) depth in the middle varying from O to 3,000 water involved are large (from 1012 to 1013 cu ,. m during an evolution lasting .PmL>u nuAQw... ..211<*T-! years m for a period of between 40 and 150 million and (3) during this evolution, successive years). deposits of sand and clay strata occurred with gradual expulsion of water from the clay as These results may provide useful indica- the result of burial. tions for studying some of the phenomena governing the formation of petroleum accumula- This basin structure is used to study the tions, i.e., diagnesis and hydrocarbon hydrodynamic conditionsthat have occurred in migration. the different sedimentarystrata. The main conclusionsof this study are as follows. INTRODUCTION ? -b. Fhlid circIdatiQnsthrowwh the clay Hydrocarbon displacementphenomena inside are quite complex because the direction of flow stratigraphicseries (primry”wl~gra.+v..“ ++nm {n_- the has usually reversed itself, with this reversal --=nln-ce— -- -reek. end secondarymigration in occurring earlier Gr A..e.7.+ - ..--...-.=fnllnwincr t,he moment permeable layers where traps-are located) caus of deposition of clayey material. This leads the sizable accumiations making up ex@oitable to an evaluationvs time of the cumulative oil fields. The leading mechanisms assumed to amounts of water, res ectively, running upward be behind these displacementsare displacemen -(WI)and downward (W2Y over a 1 sqcm of hydrocarbonsin a dissolved or emulsion horizontal surface associated with the clay state in water and displacementof continuous deposited in a given place at a given- mumeuu.— .—.- 4. oil and gas phases (or possibly oil dissolved in gas) by gravity or aquifer movement. 2. The circulationof water through clay causes fluid overpressurescompared with the It is hoped that an understandingof the hydrostaticpressure. The vertical distribu- hydrodynamic conditions existing in a sedi- mentary basin during its evolution will allow References and illustrationsat end of paper. STUDY OF THE HYDRODYNAMICPATTERN IN A SEDIMENTARY —.——. 2 BASIN SUBJEET W SUBSIDENCE sm 29Ew definition of the physical or chemical mecha- Laws of Evolution nism by which such migrations occur. There- fore, we shall attempt to reconstitutethese Sedimentationis assumed to have occurred hydrodynamic conditionsby comput@. Our underneath a water depth which is unifo?xnin reasoning will be based on a basin pattern space but does not need to be defined nor even whose structure and evolution are defined from assumed constant in time. what is known about sedimentarybasins and from hypotheses (choosingones that are as simple The sand is assumed to be an incompress- as possible) concerningwhat is poorly known. ible homogeneousmaterial with constant sand Although such hypotheses,like the approxi- layer porosity and thiclmess. mations that may be made during the project, may appear to be entirely justified,and even The clay is assumed to be compressibleand though computingmay provide results with an at all times to have a porosity dIat a given Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/70FM/All-70FM/SPE-2988-MS/2069932/spe-2988-ms.pdf by guest on 28 September 2021 nr.rm~vsow wh< -h Aenanrlc ~~~~ay ~p. ~~.~ ~~t,~ ~~- no-in+..wi+.h +.hi .- nnvnsi+.v rltaneniiinu nnlv op. ~.~.~ -“.- -“J ....*”.. -“y”..-” =-—.. , ..-”-- “-.-w y-. -W . -vy”..-_. ~ ----~ voted to this operation, these results are depth y of this point. The relation between still nothing more than an order of magnitude. porosity and depth is 0= l/(L.y+A),with FACTS OF THE PRf)BIJ!l!ANDBASIC HYPOTHESES =Oogfor y=O = 0.2for y= 1,000m. Shape, Structure and Evolution of the Basin (Fig. 1) This equation is suggestedby bibliographic data. Compression of the clay leads to the Reasoning is based on a sedimentarybasin that expulsion of the fluid filling up the clay has undergone subsidence. pores. This fluid mainly consists of a water phase possibly contax hydrocarbons (in a Shape and Dimensions form that we will not fix) in small amounts. The characteristicsof this fluid will be taken The basin consideredis in the form of an up later on. inverted cone with a circular base having a radius R = 300 km. The depth in the center, It is assumed that solid clay material is H, varies with time from O to 3,000 m at the added to the center of the basin, with a terminal state of an evolu ion correspondingtc constant deposit rate in time (expressedin k a total duration of 150.10 years. It can be mass per time unit) snd constant character- seen that this is a very flat structure [(R/H) istics of the deposited material (porosity, ~ 100] and that the representativediagrams grain size, morphology). use two very different scales for the horizon- tal and vertical dimensions. It is assumed Laws Governing Fluid Flow that the. ..w -—=l]rfa~e--”w nf-- +.he. ..” a-Lwmmd---- .-.Pan he-. =mlat.d.=—-.- to a plane. Thro~h Sand Structure of the Basin The permeabilityof the sand is assumed to be constant and identical at all points in The sedimentary series filling the basin the sand material. is assumed to be made of two parts which will be called “clay” and “sand”. The clay Through Clay represents the source rock for the hydro- carbons. It is compressibleand has low Flow through the clay is assumed to follow permeability. It was through this rock that Darcy’s law (for a possible criticism of this primary migration took place. The sand hypothesis,Cf. 2). The permeability of the represents permeable and not very compressible clay material is assumed to depend only on the 1 . ----- :...-:A- -s . ..L< ,.L . ..A-A.-. .“.< -t...+< 6.- J_dyC~b JJ1=4UG UL W1lLGL1 =CGUJM=lJ lLl*~J-cLb*UA1 porosity according to a relation of the type took place. They may form reservoirs in place~ K = A .05 (Cf. 2), with K being the perme- where their structure is suitable for hydro- ability and k being a constant. carbon accumulations. Fluid Characteristics With regard to the relative arrangement of the sand and clay parts inside the series, The fluid circulatingin the sand and clay it is considered to be a single sand layer is consideredto be in a monophasic state (or originally deposited on an impermeablebase- at least that it makes up a phase whose satu- ment with a constant thickness and covered by ration at all points is very close to 1 so that clay. the correspondingrelative permeability can also be assumed equal to 1). The fluid is assumed to be similar to a simple viscous fluid, with its viscosity at any given point SPE 2988 C. JACQUIN and M. J. POULET 3 depending only on the local temperature. Fig. Depth-Time Relationship 2 shows the relationshipexisting between these two parameters. The temperaturedepends The law governing the evolution of depth linearly on depth as defined by the following with the in the center of the basin is first two conditions: (1) surface temperature = 20° determined. At a given moment the shape of C and (2) a temperatureincrease of 3°C for the basin is thus lmown, along with the dis- each depth increase of 100 m (correspondingto tribution of porosity values at sll points in a mean value of the geothermal gradient). the clay, as well as the rate of burial. Fig. 3 shows the graphic representationof how deptk The other factors capable of changing the evolves with time in the center of the basin. viscosity values of the fluid are pressure and salinity. A study of Fig. 2 with regard The rate of burial (slope of the curve in Downloaded from http://onepetro.org/SPEATCE/proceedings-pdf/70FM/All-70FM/SPE-2988-MS/2069932/spe-2988-ms.pdf by guest on 28 September 2021 to s~~ty and Fig. 253.1 in Ref. 3 with Fig. 3) can be seen to reach a maximum at the regard to pressure shows that the possible initial moment and to become almost constant role of these two factors is entirely negli- once the depth reac es 1,000 m, which corre- gible in the case with which we are concerned.