As a source rock begins to generate oil and gas, the generated HC fluid cannot just leave the source right away, it will first need to saturate the kerogen's adsorption capacity, which depends on the total organic carbon (TOC). The volume retained by adsorption can be a significant of the generative potential - 20% for a good source rock, and 50% or more for a poor one. This can be estimated by the extract of petroleum/bitumen in the source rock. After that, additional volume generated may be trapped within the pore systems (especially if the source is heterogeneous - with interbeds of shales, limestone, marls, and silty interbeds) of the source rock as we see in shales that we produce from, and this can be very significant, also on the order of up to 50% of the generative potential of the source. There is no secondary migration up to the time until the saturation induced capillary pressure is high enough to allow primary migration out of the source rock.
Secondary migration first occurs in "first carrier beds", which are layers of more porous beds directly above, or below, or interbedded with the source. In the carrier beds, it forms accumulations in structure (3, 4 ways) and stratigraphic traps, large and small (down to pore scales), that need to be filled before migration continues, either leaking up wards, or spilling out side of the mature kitchen. Since this happens near the source rock, the lateral extent is as large as the kitchen/fetch area. This consumes a large volume, up to 100% of the generated volumes, and the time it took to generate this additional volume.
There can be additional carrier beds, and large and small traps that need to be filled before HC fluids finally reach our target trap. All of these cause the delay of charging the traps we want to drill. This delay/lag is a function of the volume of all of these traps (also called hotels, motels) between the source rock and the target trap. This lag explains in many basins where oil is found in traps that formed up to 10s of millions of years after oil generation occurred, such as the oil fields in the West of Shetlands basin:
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Typical burial history of of the kitchen area for the Faroe-Shetlands basin. Despite oil generation from the Jurassic source rock (green start) occurring mainly in Cretaceous time, the lower Tertiary reservoirs (yellow start) are filled with low GOR black oils (eg. the Foinaven and Schiehallion fields). This was explained as migration delayed by first "moteling" in deeper traps, Scotchman et all 2006. |
HC migration does not stop when the source rock is exhausted as we might expect. This is because the volume of HC fluids trapped in these deeper traps (hotels) continue to mature - cracking from larger molecules to smaller ones, and gas oil ratios (GOR) continue to increase. This volume increase can be larger the the volume generated by the source rock. Continued compaction, diagenesis also reduce the size of these hoteling traps, and cause additional migration. This is why we are seeing very young traps being filled very recently, long after the source rock is spent.
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| Schematic explanation of migration lag. Note that the present day condition of a basin/area could be at one of these stages. The target shallow trap has not been charged yet if the system is at stage 2 at present day, although the source rock is mature. It should be obvious that the hoteling traps directly above the source should be the main targets in all stages. |
Some times, or should I say very often, the hoteling traps are so numerous, or large that generated volume is entirely consumed by them, and the traps above them we are targeting don't get charge at all. One very useful observation, globally, is that exploration targeting the hoteling traps (first carrier beds) are very successful - in fact - 80% of more of the world's petroleum reserves are found in these (Lower Cretaceous reservoirs of western Siberia, Jurassic/Cretaceous reservoirs in the Middle east, Middle Jurassic in the North Sea below the source rock, Wilcox play in deep water GoM ... ..., ). Success rate exponentially decreases for traps further up stratigraphy.
Unconventional plays are essentially the hotels that we are now and producing from. East Texas field, Giddings Field are conventional reservoirs and the oil was generated and migrated from the Eagle Ford. The Eagle Ford retained about 15 mmbls/km2 of oil, which would have to be filled first before migration toward the conventional fields happened. The Woodford, Meramec would have to be filled before oil could migrate northward to North Oklahoma and Kansas.
Note that the original notion of hoteling/moteling may imply that some tectonic movement is necessary for the hotels to spill at a later time. This can happen of course, but more generically it is not necessary. Generation and cracking to lighter fluids is continuous, and volume expansion of HC trapped near the source is continuous, once a hoteling trap is filled, it will continue to leak/spill, porosity loss in the hoteling traps continues, all as long as the burial continues. In rift basins like the FSB, the thermal subsidence and the associated tilting toward the basin center, therefore spilling up-dip, is continuous too.
More generally speaking, every trap is a hotel, while being filled, it causes a migration lag for the next trap in the chain of spilling or leaking. We can only drill and produce economically a limited number traps in a basin. The deeper, non-economical ones are then referred to as hotels, or migration losses. But they are all over the kitchen area and contain much more volumes. In some shallower basins we are drilling and producing from traps near the source and the source rock itself (unconventional). The Eagle Ford contains more oil and gas that ever been found in conventional traps sourced from it. Back in time, these conventional traps up dip had to wait until Eagle Ford itself was filled. The distribution of HC volumes in a basin is a pyramid stratigraphically speaking. The base is the source rock, and the bottom 1000 meters typically contains more than half the volume, and often > 90%.
Further reading:
https://www.searchanddiscovery.com/pdfz/documents/2017/42014he/ndx_he.pdf.html
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