Small-scaled lateral variations of an organic-rich formation in a ramp-type depositional environment (the Late Jurassic of the Boulonnais, France): impact of the clastic supply

We studied a potential petroleum source rock deposited in a clastic-dominated ramp environment: the Argiles de Chatillon Formation (Kimmeridgian-Tithonian, Boulonnais area, northern France). The formation was deposited along a proximal-distal gradient on this ramp affected by synsedimentary fault movements. A sedimentological, mineralogical and geochemical study was conducted to decipher the distribution patterns of sedimentary parameters along such a depth increase over the ramp. It comes out that smectite distribution unexpectedly mimics the lateral depth evolution despite the good floatability of the mineral. It is also observed that the Argiles de Chatillon could accumulate noticeable amounts of organic matter in spite of paleo-environmental conditions that were not specifically prone to organic matter preservation and burial (sedimentation rate, mineral particle grain size, productivity, oxygenation level...). Conversely, reactive iron, when being present in limited abundance, must have allowed sulfide ions to react with organic molecules instead of forming iron sulfides, which fostered organic matter preservation and accumulation. This protecting role of organic sulfide incorporation (additional to other favorable factors) cannot exist if reactive iron is relatively abundant. Lastly, our results (still fragmentary) suggest that smectite minerals could carry reactive iron, which would have occasionally hampered organic-matter preservation. (authors)