The link between rock physics properties and sedimentary microstructure

Typically in the shallow parts of basins shales have higher AI than the sands, whereas in the deeper (more compacted) section the sands have higher AI than shales. Thus, at a certain depth the sand and shale impedance trends cross-over, with very low AI contrast between them. Above this depth, shales are harder than sands Below the cross-over, the sands are harder than the shales and the high AI (low porosity) sands start to have markedly low PR values

Rock physics properties change with the depositional environment and burial depth. These geologic trends must be taken into account during hydrocarbon prediction from seismic data.

The vertical line and ellipse depict the known area from good control in the shallow shelf edge. Arrows indicate deeper and more distal waters where we would like to predict the changes in seismic response.

Important Rock physics model related to?Diagnosis and deposition

?The friable- (unconsolidated) sand model

The friable-sand model represents velocity–porosity–sorting variation within a sand unit. For quartz-rich sands the sorting variation is due to smaller quartz grains filling into the pore space between larger quartz grains. However, deteriorating sorting is normally associated with increasing clay content, and if the clay content is relatively large (>20%) we are talking about a shaly sandstone

The contact-cement model.

The contact-cement model represents the initial stage of the “diagenetic trend” in the data. It is found to be applicable to high-porosity sands. During more severe cementation where the diagenetic cement is filling up the pore space, the contact theory breaks down because ignore sorting effect?

The constant-cement model Avthes 2000.

For a reservoir at a given depth, where the sands are consolidated, the constant cement model represents the most likely scenario. The amount of cement is often related to depth, whereas sorting is related to lateral variations in flow energy during sediment deposition. Making this assumption, we can refer to this model as a “constant-depth model” for clean sands.

Diagnosing Paleocene turbidite sands

Diagnosing Paleocene turbidite sands

Well 1, we observe a gradual variation of clay content between very clean sand and shale. Only a relatively thin (10 m) sand interval is identified as a practically clay-free reservoir sand.

?InWell 2, unlike in Well 1, a thick oil-saturated sand interval (gray bar in Figure 2.14C) is marked by extremely low and constant gamma-ray readings (about 55 GAPI) and high velocity (about 3 km/s). This sand layer is surrounded by shale packages whose gamma-ray

readings and velocity strongly contrast with those of the pay zone sand. As mentioned,

the clean sand zones in both wells represent the same stratigraphic unit, although located

at different depths and in separate oil fields.

The velocity difference between the pay zones in the wells under examination is

emphasized in , where the P-wave velocity is plotted versus porosity.

The rock diagnostic shown in Figure 2.15 implies that the sands inWell 2 have small

initial contact cementation. The porosity decrease from the initial-cement porosity is

likely to be due to deteriorating sorting (smaller grains fall in the pore space between

larger grains and have a large effect on the porosity).

We\observe that the porosity is lower and the grains more closely packed in the two lower

pictures. Thus thin-section analysis confirms that the degree of sorting varies within

the studied sand interval.

The three curves come from the contact-cement, constant-cement, and

friable-sand models. The solid is assumed to be pure quartz; the porosity of the initial

sand pack is 39%, and the initial-cement porosity φb is 37% (the latter corresponds to

contact cement occupying about 2% of the pore space of the initial sand pack).

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