R. R. Loucks
Kitty L. Milliken
Quansheng Liang
Tongwei Zhang
Xun Sun
Paul C. Hackley
Stephen C. Ruppel
Sheng Peng
Lucy T. Ko
2017
<p><span>Our main objectives are to (1) learn if pore-evolution models developed from marine mudrocks can be directly applied to lacustrine mudrocks, (2) investigate what controls the different pore types and sizes of Chang 7 organic matter (OM)-rich argillaceous mudstones of the Upper Triassic Yanchang Formation, and (3) describe the texture, fabric, mineralogy, and thermal maturity variation in the Chang 7 mudstones. Lacustrine mudstones from nine cored wells along a depositional dip in the southeastern Ordos Basin, China, were investigated. Helium porosimetry, nitrogen adsorption, and field-emission scanning electron microscopy of Ar-ion milled samples were applied. Measured average total porosity of samples from a proximal to distal transect (</span><span class="equationTd inline-formula"><span id="MathJax-Element-1-Frame" class="MathJax" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mrow><mi>&#x3D5;</mi><mo>=</mo><mn>5.0</mn><mo form="postfix">%</mo></mrow></math>"><span id="MathJax-Span-1" class="math"><span><span id="MathJax-Span-2" class="mrow"><span id="MathJax-Span-3" class="mrow"><i><span id="MathJax-Span-4" class="mi">ϕ</span></i><span id="MathJax-Span-5" class="mo">=</span><span id="MathJax-Span-6" class="mn">5.0</span><span id="MathJax-Span-7" class="mo">%</span></span></span></span></span></span></span><span>) is higher than those from the two adjacent cored wells (</span><span class="equationTd inline-formula"><span id="MathJax-Element-2-Frame" class="MathJax" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mrow><mi>&#x3D5;</mi><mo>=</mo><mn>2.3</mn><mo form="postfix">%</mo></mrow></math>"><span id="MathJax-Span-8" class="math"><span><span id="MathJax-Span-9" class="mrow"><span id="MathJax-Span-10" class="mrow"><i><span id="MathJax-Span-11" class="mi">ϕ</span></i><span id="MathJax-Span-12" class="mo">=</span><span id="MathJax-Span-13" class="mn">2.3</span><span id="MathJax-Span-14" class="mo">%</span></span></span></span></span></span></span><span>). This difference in porosity partly caused by differences in the clay mineral content implies that in the fluvial-deltaic-lacustrine depositional environment, reservoir quality can vary significantly in a short distance. Owing to the uneven distribution of the sample set from proximal to distal area, we mainly evaluate variations in the proximal setting. Results from nitrogen-gas adsorption experiments show that there are four distinct patterns of pore-size distribution within the Chang 7 member of the Yanchang Formation with no particular correlation with mineralogical composition and thermal maturity. The pore network within Chang 7 mudstones is dominated by OM-hosted pores, with a lesser abundance of interparticle and intraparticle pores. The size distribution of mineral-hosted pores within these mudstones is found to be closely related to the rock texture (sorting and grain size) and fabric. Mudstones with well-sorted grains and a higher percentage of coarser grains have more abundant mineral pores. The sizes of OM-hosted pores in these compaction-dominated lacustrine mudstones were one to two orders of magnitude smaller than those in the marine mudstones that display abundant early cementation.</span></p>
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10.1190/INT-2016-0115.1
en
Society of Exploration Geophysicists
Controls on pore types and pore-size distribution in the Upper Triassic Yanchang Formation, Ordos Basin, China: Implications for pore-evolution models of lacustrine mudrocks
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