位于致密地层（如泥岩）中的渗透性透镜体（如砂体），由于压力传递的质心效应（centroid effect），其上倾部分（A）的压力比正常地层压力高，形成超压；而下倾部分（C），则比相应的地层压力低，形成低压；中部与地层压力相交的地方，即为质心（centroid）。质心效应表明，地层超压有时并不是压实不平衡造成的。
References:
[1] England W A, Mackenzie A S, Mnn D M, Quigley T M. The movement and entrapment of petroleum fluids in the subsurface [J]. Journal of the Geological Society, 1987, 144(2): 327-347.
在高压釜中加热水淹的烃源岩样品，即加水热模拟实验（hydrous pyrolysis），反应后在水中可收集到与地下原油相似的产物。根据Lewan（1993）的实验，加水热模拟实验过程中，样品的干酪根含量逐渐降低，伴随着排油量的增大，并在350℃左右排油量达到最大，此后由于油裂解，排油量降低。与排油量对应的是残留在源岩中的沥青抽提物，沥青抽提物的含量与排油量变化趋势相同，也是先增大后减小，不过沥青抽提物的峰值先于排油量到达，在330℃左右，一般解释为干酪根先生成沥青然后生成石油（注意：沥青抽提物，如氯仿沥青“A”，与沥青的区别），其中沥青是中间产物。与源岩无水热模拟实验相比，加水热模拟实验表明，源岩的生烃在较低的温度下即可进行（无水热模拟实验一般需要500℃左右），而且可以得到类似地下原油的产物。
水在烃源岩中是普遍存在的，水对源岩的生排烃的作用不容忽视。那么水在源岩的生烃中到底起什么作用呢？Lewan（1997）的研究结果表明，水在干酪根转换成沥青的过程中（首先断开弱化学键）作用不明显，但对于沥青进一步转换成石油（断开共价键，形成自由基）却是重要的因素。在无水条件下，沥青主要发生交联作用，生成焦沥青（pyrobitumen）；而有水条件下，沥青主要发生裂解作用，生成饱和烃。Schimmelmann(1999,2001)等通过重水（D2O）示踪研究加水热模拟实验中氘的流向进而研究水的作用机理，结果表明烃源岩加水生烃过程中，普遍存在水的氢转换成有机质中的氢。这种转换程度与源岩的渗透率及源岩颗粒大小有关；渗透率越大，颗粒越小，水与源岩的接触面积越大，这种转化程度也越高。相对其它有机质类型，Ⅰ型干酪根主要为烷基链，不易形成自由基，因此Ⅰ型有机质生成的油中含有的来自水的氢的含量最低，其余几种类型有机质生成的石油中的来自水的氢的比例依次增加：Ⅱ≈Ⅲ>ⅡS。
启示：水中的氢在生烃过程中可以转换成有机氢，因此，对于一些贫氢的有机质的生烃作用，我们可能需要重新审视。
主要参考文献：
[1]Arndt Schimmelmann, Jean-Paul Boudou, M.D Lewan and etc. (2001) Experimental controls on D/H and 13C/12C ratios of kerogen bitumen and oil during hydrous pyrolysis.Org Geochem, 32(8):1009-1018.
[2]M.D Lewan. (1993)Laboratory simulation of petroleum formation: hydrous pyrolysis. Org Geochem, New York 1993,419-442.
[3]M.D Lewan. (1997)Experiments on the role of water in petroleum formation. Geochimica et Cosmochimica Acta, 61(17), 3691-3723.
烃源岩评价方面(source rock evaluation)：
1. Mackenzie,et al. Principle of geochemical prospect appraisal,AAPG Bull,1988,72(4):399-415.
2. Mackenzie,et al. A novel approach for recognition and quantification of hydrocarbon migration effects in shale-sandstone sequences. AAPG Bull,1984,68(2):196-219.
3. Harry Dembicki, JR.,et al. Source rock evaluation by pyrolysis-gas chromatography, AAPG Bull,1983,67(7):1094-1103.
4.Bishop, et al.Concepts for estimating hydrocarbon accumulation and dispersion, AAPG Bull,1983,67(3):337-348.
5.Cooles,et al. Calculation of petroleum masses generated and expelled from source rocks, Org. Geochem.,1986,10:235-245.
6.Lewan.Experiments on the role of water in petroleum formation,GCA,1997,61(17):3691-3723.
7.Katz, Limitations of ‘rock-eval’ pyrolysis for typing organic matter, Org. Geochem.,1983,4(3/4):195-199.
8.Peters, Guidelines for evaluation petroleum source rock using programmed pyrolysis, AAPG Bull,1986,70(3):318-329.
9.Landford, et al. Interpreting rock-eval pyrolysis data using graphs of pyrolizable hydrocarbons vs. total organic carbon,1990,74(6):799-804.
10.Conford, et al. Geochemical truths in large data sets. I: Geochemical screening data, Org. Geochem,1998,29(1-3):519-530.
11.Espitalie,et al. Role of mineral matrix in kerogen pyrolysis: influence on petroleum generation and migration,1980,64(1):59-66.
12.Dembicki, Three common source rock evaluation errors made by geologists during prospect or play appraisals, AAPG Bull,2009,93(3):341-356.
待续…
Clayton, C. (1991). “Carbon isotope fractionation during natural gas generation from kerogen.” Marine and Petroleum Geology 8(2): 232-240.
In petroleum exploration it is important to be able to determine the origin of any gas which is found. This paper describes a new method of estimating the source-type and maturity of a gas based on a Rayleigh fractionation model. Kerogen is divided conceptually into a labile (dominantly oil-generating) fraction and a refractory, gas-prone, component. [delta]13C of methane from either kerogen type, and ethane, propane and butane for gases from labile kerogen, can be defined as a function of [delta]13C of the gas precursor groups in kerogen, a kinetic isotope fractionation factor, k, and the extent of gas generation. The isotopic ratio of the methane precursors relative to bulk kerogen, determined from laboratory pyrolysis, are -17.5[per mille sign] for labile kerogen and -1.4[per mille sign] for refractory kerogen. Values for ethane, propane and butane from labile kerogen, based on field correlations, are -4.9[per mille sign], -2.2[per mille sign] and -1.6[per mille sign] respectively. The corresponding fractionation factors are 0.9892, 0.9919, 0.9947 and 0.9975 for methane, ethane, propane and butane respectively from labile kerogen, and 0.9984 for methane from refractory kerogen. Using these parameters, summary diagrams are constructed which allow differentiation of these sources from each other and from biogenic gases and cracked oil, and recognition of gases of mixed origin. If an independent estimate of [delta]13C for the source kerogen is possible, then [delta]13C of the gas components can be used to estimate maturity in terms of the Gas Generation Index, the fraction of gas potential which has been realized.
Cramer, B., B. M. Krooss, et al. (1998). “Modelling isotope fractionation during primary cracking of natural gas: a reaction kinetic approach.” Chemical Geology 149(3-4): 235-250.
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