Sedimentary environment of organic-rich shale in the Upper Permian Longtan Formation in Qi

China?Geology?1?(2019)?108?109

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China?Geology

Journal?homepage:?http://chinageology.cgs.cn

Research?Advances?

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Sedimentary?environment?of?organic-rich?shale?in?the?Upper?Permian?LongtanFormation?in?Qinglong?area,?western?Guizhou,?China

Rong?Chen*,?Kun?Yuan,?Zi-ya?Zhang,?Qiu-feng?Xu

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Oil and Gas Survey, China Geological Survey, Ministry of Natural Resources, Beijing 100083, China

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1. Objective

The?Qinglong?area?is?located?in?the?Eastern?Yunnan-Western?Guizhou?uplift?tectonic?belt,?west?of?the?Zishui?faultzone?and?north?of?the?Youjiang?basin.?There?are?many?sets?oforganic-rich?shale?in?Longtan?Formation?in?Qinglong?area.Many?predecessors?did?a?great?deal?of?research?on?thegeological?features?of?Permian?coal?fields?in?Western?Guizhou(Mei?MX?et?al.,?2004;?Dou?XZ?et?al.,?2012),?but?lack?of?theresearch?on?sedimentary?background?and?distributioncharacteristics?of?organic-rich?shale?in?Longtan?Formation.Therefore,?this?study?focuses?on?the?analysis?of?thesedimentary?environment?of?the?Upper?Permian?LongtanFormation?in?Qinglong?area.2. Methods

The?distribution?characteristics?and?sedimentaryenvironment?of?shale?are?the?important?basis?for?shale?gasexploration?(Zhai?GY?et?al.,?2018).?Based?on?the?analysis?ofsedimentary?facies?of?typical?profiles?and?wells,?thesedimentary?facies?were?analyzed?and?six?sections?(wells)?inQinglong?and?its?surrounding?area?have?been?contrastedaccording?to?rock?type,?mudstone?color?and?biodistribution.The?sedimentary?environment?of?Qinglong?area?in?the?latePermian?is?discussed.3. Results

According?to?the?lateral?comparison,?the?depositionalthickness?of?the?Upper?Permian?Longtan?Formation?in?thenorthern?part?of?Qinglong?County?is?the?thickest,?and?in?thecentral?part?is?the?thinnest.?The?sedimentation?thickness?of?the

Longtan?Formation?increases?from?south?to?north.?TheLongtan?Formation?in?Qinglong?and?its?peripheral?area?can?bedivided?into?three?sequences?and?five?system?fields?(Fig.?1).Sequence?I?developed?transgressive?systems?tract?andhighstand?system?tract?(Fig.?1).?There?are?one?or?morebiogenic?limestone?layers?in?the?transgressive?system?tract.The?biological?fossils?in?sequence?I?shows?that?sedimentaryfacies?were?tidal?flat?and?lagoon?facies.?In?the?high?systemtract,?there?are?mostly?mudstone?and?siltstone?with?one?ormore?sets?of?coal?seam,?showing?the?deepening?of?waterbodies.?In?Qinglong?area,?the?sedimentary?facies?of?SequenceI?were?mainly?shunt?bay,?swamp,?tidal?flat?and?lagoon?facies(Fig.?2a).?During?this?period,?the?Qinglong?area?was?still?in?arelatively?deep-water?environment.

In?sequence?II,?the?transgressive?system?tract?and?thehighstand?system?tract?are?developed.?The?northern?part?ofQinglong?was?the?delta?front?facies?with?deltas?developed,?andgradually?transformed?into?the?former?delta?facies?and?thetidal?flat-lagoon?facies?(Fig.?2b).?During?this?period,?the?waterwas?shallower?from?south?to?north.?The?delta?appearing?in?thenorth?shows?that?the?source?may?come?from?the?north.

Sequence?III?developed?transgressive?system?tract.?Thewells?in?the?northern?area?are?mainly?composed?of?siltstoneand?thin-bedded?mudstone.?In?the?southern?area,?sequence?IIIdeveloped?siltstone,?mudstone,?coal?seam?and?argillaceouslimestone,?the?layers?of?Limestone?are?more?than?the?northernarea.?The?sedimentary?facies?of?northwestern?Qinglongconvert?to?shunt?bay?of?the?delta?plain?(Fig.?2c).?Thesouthwestern?Qinglong?remained?deep?water?basin?butcompare?to?sequence?II?the?depth?of?water?was?muchshallower.4. Conclusion

The?Longtan?Formation?of?the?Upper?Permian?in?theQinglong?area?can?be?divided?into?three?second-ordersequences?and?five?system?tracts.?The?early?depositional

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?*?Corresponding?author:?E-mail address:?akihisa@qq.com?(Rong?Chen).

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doi:10.31035/cg2018086

2096-5192/??2019?China?Geology?Editorial?Office.